John Stachel is a person with a wide range of interests and a broad sense of humor. Inthinking about a suitable contribution for this volume, I recalled that about a decadeago I was asked to contribute an essay (Janis 1989) on astronomy and science fictionto a volume in memory of the astronomer M.K.V. Bappu . (This invitation camebecause , for many years, I taught a course for non-science students in which I usedscience fiction as the setting for teaching many aspects of physics and astronomy.) Itoccurred to me that an essay on physics and science fiction, although undoubtedlyrather different from the usual sort of f estschrifl contribution, might be a pleasantchange of pace. My earlier essay on astronomy and science fiction assumed little, ifany, scientific background. For the present volume, given its more technical but stillinterdisciplinary character, I have tried to write at a level that assumes some technicalbackground but is still comprehensible to non-specialists . At any rate, it has been funfor me to write this essay, and I hope that John, as well as other readers, will have funreading it.
Since a comprehensive treatment of physics and science fiction would fill (atleast) an entire book, it is necessary to limit the scope of this essay. After some briefremarks (in section 2) about the different ways in which science appears in sciencefiction, I shall (in section 3) describe the use (and sometimes misuse) of physics in asmall selection of science fiction stories ; they are all stories that I have used in teaching. Finally (in section 4) I shall make some further comments on the use of sciencefiction as a teaching tool.
It is a pleasure for me to offer this essay for John Stachel 's enjoyment as part ofthis celebration of his 70th birthday.
2. THE SCIENCE IN SCIENCE FICTION
In terms of their use of science, science fiction stories can differ widely. The storiesthat are usually the most fun for scientists to read are those that use sound scientificideas in imaginative ways. Sometimes known scientific principles will be deliberatelyignored in order to make a story possible (for example, by postulating various waysof traveling faster than light), but the other aspects of the story will strictly respect theknown laws ofnature. Sometimes a story that is almost entirely good science will slip
up at some point and allow an error to slip in. Other stories are so full of errors thatone can believe that they are called science fiction only because the science in them isfictitious. Even such a story, within a classroom setting, can sometimes be a usefulway to introduce students to scientific ideas.
Some writers of science fiction are excellent scientists themselves. The best ofsuch authors will sometimes use well established scientific principles in surprisingand delightfully imaginative ways, and sometimes go beyond the bounds of knownscience to speculate in ways that nevertheless have sound underpinnings. Some haveeven been known to put in a plug for a favorite, but controversial, theory (see the nextsection).
Science fiction, of course, is not written solely as a showcase for scientific ideas.Even those stories with the best use of science have, like all literature , a variety ofaims, some light-hearted (e.g., humor), others serious (e.g., social or political commentary). Some of my comments on specific stories will include remarks along theselines.
3. SOME STORIES (AND SOME ERRORS)
Larry Niven's short story Neutron Star (1966) contains much good science. In brief,the protagonist, Beowulf Schaeffer (a character appearing in a number ofNiven's stories, and one whose personal characteristics suggest that his initials may not havebeen chosen entirely at random), has been blackmailed into piloting a spaceship tothe vicinity ofa neutron star in order to discover what killed Sonya and Peter Laskin,who had piloted a similar ship there. The makers of the ship had a reputation to maintain: their ships were guaranteed to be impervious to everything except visible light,but something had wrecked much of the ship's interior and reduced the Laskins to acollection of bloody smears. Schaeffer discovers the answer: intense tidal forces nearthe neutron star ripped things apart, throwing virtually everything inside the ship intoone end or the other with great violence. Schaeffer escapes with his life by staying atthe center of mass.
Niven correctly describes many aspects of the physics of strong gravitationalfields: gravitational bending of light, including Einstein rings; the gravitational frequency shift, which in the case of Schaeffer observing distant stars from the vicinityof a neutron star is a blue shift; and, of course, strong tidal effects, including the tidallocking of the ship into a position with its axis pointing toward the star. However,Schaeffer would not have survived the trip.
I have been told that Niven later found a problem with the story. Since the tidalforces caused the ship's axis to line up with the neutron star, the ship's rapid passageby the star would have imparted such a high angular momentum to the ship that itwould not have been safe for Schaeffer to leave the center of mass-had he done so,the rapid spin would have smashed him into one of the ends of the ship. But there isanother calculation that Niven apparently never did.
Let us try to estimate the tidal gradients that Schaeffer encountered . According todata given in the story, the star had a mass of 1.3 solar masses and a diameter ofabout12 miles , and the ship 's closest approach to the surface was about one mile. Since this
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means that the ship came within 3 Schwarzschild radii of the star 's center, the effectsof general relativity would certainly not be negligible. Nevertheless, just to get a veryrough approximation, let us see what Newtonian physics would say.
From the Newtonian expressionGM/r2 for the strength of the gravitational field(or, equivalently, the acceleration of a freely falling test mass) produced by a star ofmassM at a distancer from its center, it follows that the difference in fields at twopoints Ar apart (where Ar is small compared to r) has the magnitude(2GM/r3 )~r .
If we evaluate this at periastron and express the answer in terms of g's (one g beingthe acceleration of gravity at the Earth 's surface) , we get approximately2.5 x 107 x Sr g's , whereAr is measured in meters . This means that ifSchaeffer could curl up into a diameter of one meter, the difference in the gravitational field strength across his body would be 25 million g's (curling up a bit smallerwould not help much) . Even allowing for the poor approximation, it seems likely thattidal forces would have ripped Schaeffer apart .'
The title 'character' in Fred Hoyle's novel The Black Cloud (1957) is a large (bigenough to enclose the Sun and cause catastrophic cooling on Earth) cloud possessingsuperhuman intelligence. This is certainly one of the most creative conceptions ofextraterrestrial life to be found. Hoyle goes into some detail as to how the cloudmight be organized. Basically, different parts of the cloud are linked by radio wavesinto a single neurological entity. Electrical discharges in its outer parts cause sufficient ionization to prevent disruption by unwanted external signal s, much as ourskulls protect our brains . An electromagnetic pump maintains a flow of gas that actsanalogously to a blood supply and passes through a filter that acts analogously to kidneys . Overall, the cloud controls vast amounts of energy, but sometimes needs to berecharged by absorbing energy from a star; hence its excursion into our solar system.When communication is established between the cloud and a group of terrestrial scientists, the cloud expresses surprise (p. 149):
for it is most unusual to find animals with technical skills inhabit ing planets , which are inthe nature of extreme outposts of life.
Hoyle's treatment of the effects of the cloud's blocking of the Sun makes gooduse of thermodynamical and statistical mechanical concepts. Much of what he foresees reminds one of the discussions of 'nuclear winter' that came much later, but the'cloud's winter' was even more extreme. The book also makes good use of a numberof other concepts of physics and astronomy, making this an enjoyable novel for readers that like science fiction containing lots of good science imbedded in a good story.One of Hoyle's predictions in this novel , however, did not tum out to be right.
Prior to close-range studies of the Moon's surface , there was considerable debateas to whether the Moon was covered by dust. The arguments were fueled by observations of the Moon's rate of cooling during lunar eclipses. There was even speculationthat the dust would be deep enough to engulf any ship that landed on the Moon.Hoyle 's views on the subject are made clear in The Black Cloud (p. 105):
The existence of vast drifts of dust on the Moon was confirmed in dramatic fashion .
As the cloud approached the Sun, it slowed down by ejecting gas at high speed.
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Some of this gas hit the Moon, dramatically changing its appearance as seen througha telescope. Hoyle has one of the astronomers explain (p. 106):
Those dark areas are gigantic drifts of dust, drifts perhaps two or three miles deep. Whatis happening is that the high speed gas is causing the dust to be squirted hundreds ofmiles upwards from the surface of the Moon.
Hoyle also includes a plug for his favored, steady-state cosmology. This comesduring a discussion between a human and the cloud about the cloud's reproductiveabilities. The cloud explains (p. 156):
If I, for instance, were to find a suitable cloud not already endowed with life I wouldplant a comparatively simple neurological structure within it. This would be a structurethat I myselfhad built, a part of myself.
The cloud further explains that it would endow its ' infant' with an electromagneticscreen to prevent radioactive materials from penetrating its neurological regions. Itcontinues:
The point of this example is that we can provide our ' infants' both with screens and withthe intelligence to operate them, whereas it would be most improbable that such screenswould develop in the course of a spontaneous origin oflife .
When the human interposes:
But it must have happened when the first member of your species arose
...the cloud replies...
I would not agree that there ever was a ' first' member.
At this point, two of the astronomers listening in
...exchanged a glance as if to say: "Oh-ho, there we go. That's one in the eyes for theexploding-universe boys."
A short story that makes clever use of standard physics is David Brin's Tank-FarmDynamo (1983). The tank farm of the title consists of two parallel flat 'decks' connected by six very long, parallel cables. The decks are composed of rows of giant cylinders covered by aluminum plating on the sides facing each other. The farm is inEarth orbit, and tidal effects create a small amount of 'a rtificial gravity ' at each end,directed away from the center of mass (as in Neutron Star, but of course smaller by anenormous factor). An elevator connects the decks, and the apparent gravitational fieldis enough to allow farming to take place in the tanks and to allow the decks to be usedfor spacecraft landings and takeoffs. In fact, the farm is used to give a boost to spacecraft that land on the deck closer to Earth, are carried to the farther deck, and then slipoff into higher orbits. There is a small engine to enable the farm to counteract theinevitable orbital decay. The tanks that make up the ever growing farm are cast-offfuel tanks from spacecraft. The residual fuel in these tanks-oxygen and hydrogen,water in effect- provides life support and fuel for the farm's engine. And now thegovernment is threatening to cut back severely on this supply of water. While a delegation from Earth is at the farm to deliver an ultimatum, the farm's scientists come upwith the solution to their problem. They use solar power to force a current through the
PHYSICS AND SCIENCE FICTION 549
tethering cables in the direction that, in the Earth's magnetic field, will feed energyinto their orbit. The bad guys, who wanted to drive the farm out of business, are leftsputtering.
Earlier in the story, the farm's scientists (or, more accurately, Brin, who has aPh.D. in Applied Physics and Space Science) realized that although their motionthrough the Earth 's magnetic field would generate an electromotive force across thecables, they could not draw power from this without further degrading their orbit.This follows from Lenz's Law, which says that the induced current would flow in thedirection that would cause magnetic drag. Whether this was understood in real life bythe relevant scientists at the American space agency, NASA, has been questioned. Asdescribed by Robert Park (1998,5):
[I]n 1992, NASA attempted to deploy a small satellite from the shuttle Atlantis tetheredby a 20-km wire . The plan was that the conductor moving through Earth's magnetic fieldwould generate electric power for the spacecraft. The mission manager described this as' a free lunch ' .
One of the tank farm's scientists comments when a similar scheme is proposed in thestory (Brin 1983, Ill):
You know there ain 't no such thing as a free lunch.
After mentioning Lenz 's Law, Park continues:
To maintain its orbit the spacecra ft would have to fire its rockets. In effect, the electricitywould be generated by the rockets-and not very efficiently. In any case , the reeljammed ...Incredibl y, NASA tried to refly the $1B mission four years later. This time thetether broke. Fortunately, NASA seems to have given up.
However, ifNASA's intention had been to convert orbital energy to electrical energy,as suggested by Geoffrey Landis (1999, 2) in a reply to Park, then the criticism wouldbe unjustified.
Tank-Farm Dynamo reminds me of Isaac Asimov's story The Martian Way(1952), in which a demagogue on Earth stirs up the people to support cutting off thesupply of water to a colony on Mars. This, too, is a story making good use of physics ;conservation of momentum , for example, is clearly explained and illustrated. In thisstory, a heroic expedition to bring ice back from the rings of Saturn saves the peopleon Mars and leaves the demagogue looking foolish. Asimov (1973, II) has explainedthat this story represented his reaction to the McCarthy era in American politics.
Ross Rocklynne's story The Men and the Mirror (1938) is an interstellar cop-androbber chase. It is one of a series in which the officer and the outlaw (the cleverer ofthe two) find that the chase has put them both in a dangerous situation from whichthey ultimately escape by clever use of the laws of physics. In The Men and the Mirror, they have landed on a planet in which is embedded a huge concave mirror,approximately a thousand miles in diameter with a depth of about three hundredmiles, and they have, unfortunately, fallen into it. The mirror 's surface is almost frictionless, but 'almost' is part of their problem: There is just enough friction to keepthem from rising back to the rim and effect their escape. Their problem is two-fold:not only how to get out but, if they succeed, to do so somewhere in the vicinity of
550 ALLEN I. JANIS
their ships. The answer to the first part comes from use of the conservation laws forangular momentum and energy. The two men are connected by a rope. Havingacquired some angular momentum about their common center of mass, they haul inon the rope, decreasing their moment of inertia about their center of mass. Conservation of angular momentum then dictates that their angular velocity must increase.They shorten their separation until they have acquired sufficient kinetic energy forescape. At a moment when they are at the apex of one of their oscillations across themirror and when in the course of their rotation one of them is aimed directly at theclosest point of the rim, they cut the rope. The one aimed at the closest rim shootsover it and out of the mirror. Conservation of energy guarantees that, if the oneescapes in this fashion, the other will similarly escape on the other side (although,due to the planet's rotation, not directly opposite) provided that the kinetic energy ofeach when the rope is cut exceeds that needed for escape by at least the amount thateach loses to friction. The solution to making one of them come out near their shipscomes from an analysis of their oscillatory motion in relation to the rotation of theplanet. Careful observations allow them to time their escape so that the second oneout is near their ships. (There was good reason for making the second one out the oneto be near the ships. The first one out would have lost less kinetic energy to frictionsince the cutting of the rope, and thus would be more likely to suffer injury whenlanding. In the story, the first one out breaks a leg, while the second one emergesunscathed and brings his ship to the rescue of the first.) Unfortunatel y, the solutionsto both parts of the problem seem to be flawed.
The problem with the first part is the same conservation of angular momentumthat the men used to escape from the mirror. The story seems to make clear that theyhad little, if any, angular momentum to begin with. It is by flailing about that theybuild up their rotational motion. In the absence of friction, conservation of angularmomentum would forbid this. It is true that the presence of friction is important in thestory. It makes their rise less with each oscillation, and provides a time constraint: Ifthey do not effect their escape soon enough, they will no longer rise high enough(with correspondingly low enough escape velocity from that height) to be able tobuild up escape velocity as they did. But the smallness of the friction seems incompatible with the rapidity with which they build up their angular momentum .
As for the timing of their escape, the analysis presented in the story has themmoving like a plane pendulum while the planet rotates under them (the axis of themirror coincides with the rotational axis of the planet); in effect, they are a Foucaultpendulum at one of the planet 's poles. However, when they fell into the mirror, theywere at rest with respect to the planet (at a point off the rotational axis, of course,since they were at the mirror's rim), not at rest in the non-rotating frame of reference .Their motion would not be that of a plane pendulum.
Arthur C. Clarke 's novel Rendezvous with Rama (1973) is a good story with lotsof good physics . Rama is an enormous , mostly hollow cylinder, 50 km long and 20km in diameter, rotating at just the right speed to produce an apparent gravitationalfield of about 1 g for someone standing on the inside surface. Rama has penetratedthe solar system, and has been spotted by a monitoring program that was set up afterthe catastrophic collision of a thousand-ton meteorite with Earth in 2077; it is now
PHYSICS AND SCIENCE FICTION 551
2131. Rama is clearly a manufactured object, the first indication of intelligence elsewhere in the Universe, but it shows no signs ofactivity. An exploratory mission is dispatched, which lands on one of the end faces and enters the cylinder along its axis.Although there are structures inside, mostly along the inside cylindrical surface, theexplorers find no signs of activity nor sources of light (other than the ones they bringin themselves) . But Rama warms up as it gets closer to the Sun, and things begin tohappen. Lights come on, a circular band of ice going around the inside of the cylindermelts (this encircling body of water is called the Cylindrical Sea), and robotic creatures appear.
Rama provides an excellent setting for exploring the physics of rotating referenceframes. The apparent gravitational field, already mentioned above, is one example ofthis. The terrestrial explorers find they can get 'down' to what they call the Raman'plain' (in spite of its curvature) by means of huge staircases built into the end facewhere they entered. These staircases change gradually from ladder-like to stair-like asthey descend and the apparent gravitational field increases. Coriolis force gets itsshare of attention also. As Rama warms up, atmospheric currents are created and areaffected by Coriolis forces, ultimately resulting in hurricanes that cause the explorersto withdraw until a new equilibrium has been established . There is also a waterfallcascading from one of the end faces that, due to the Coriolis effect, lands many kilometers to the side of the point directly below its source.
Another interesting point of physics relates the heights of the cliffs on either sideof the Cylindrical Sea to Rama 's acceleration. On one side the cliffis only 50 m high,but on the other it is ten times that. Assuming that the cliffs are meant to keep the seafrom sloshing over onto the plain, the larger cliff height and the width of the Sea areused to calculate the maximum acceleration that Rama can sustain (and, ofcourse, itsdirection) . At the end of the story, Rama is found to leave with an acceleration that is75% of this calculated maximum .
There are many other examples of the good use of a variety of physical conceptsin the story, and one intentional break from known physics: Rama's propulsion mechanism. At the end of the story, after the explorers have left Rama for good, they findtheir ship rolling in Rama's wake. They can tell they are rolling by looking at thestars, but they feel nothing (they are rolling fast enough that they would expect to feelit) and their instruments show no rotation. They conclude that only a powerful gravitational field, among the known fields, could produce such an effect. There is no evidence of any exhaust-Newton's third law seems not to have anything to do withRama 's acceleration.
Jerome Bixby 's story The Holes Around Mars (1954) provides a stark contrast toRendezvous with Rama , at least insofar as the correct use of physics is concerned.Asimov (1971, 235) introduces his own comments about this story with the words:
Jerome Bixby, although an excellent piano player, is not an author usually associatedwith 'scientific' s.f. The Holes Around Mars is entertaining and fascinating, but it hasmany scientific flaws.
A terrestrial expedition to explore Mars discovers a series of holes, apparently allat the same altitude (although one of the scientific confusions in the story is the sug-
552 ALLEN 1. JANIS
gestion that the holes are both in a perfect circle around the center of Mars and in astraight line), in anything on the Martian surface that is higher than that altitude andlying along a 'straight line' (presumably, a great circle) . By the end of the story, theydiscover that the holes have been formed by a hitherto unknown, very small (aboutfour inches in diameter) moon, which is christened Bottomos by the expedition'sleader, who has a penchant for puns (much to the disgust of his companions) . Howthis moon could punch its way through, among other things, solid rock is not satisfactorily explained. Even stranger, doing so does not seem to affect its orbit , for it goesthrough the same holes on each passage. That it frequently encounters fresh obstaclesis made clear in the story, for one of the holes discovered by the explorers is in a cactus-like plant that is still oozing from the impact; but Bottomos ' path is seeminglyunaltered by the encounter. At the end of the story, it is suggested that Bottomos initially was traveling fast enough to punch its way through such things as rocks, butthat these encounters slowed it until now it can only penetrate softer materials-andthis slowing apparently took place without any change of orbit! Another problem isthat, since the orbit does not change relative to the Martian surface, it must be equatorial, but internal evidence in the story shows that at least some of the holes are not atthe equator. Near the climax of the story, the explorers come upon a Martian village(this story was written before such a thing became totally unthinkable) . Bottomos'orbit takes it down the main street of this village a couple of feet above the ground, sothe inhabitants have a calendar posted by the street on which they keep track of thedays when Bottomos will arrive; they apparently keep track of the actual time ofarrival by checking the lengths of their shadows. But let us take a closer look at Bottomos ' orbital period.
Kepler's third law tells us that, for objects orbiting the same central body, thesquares of the periods are proportional to the cubes of the semi-major axes. Usingorbital data from the known moons of Mars, Deimos and Phobos , and taking theradius of Bottomos' orbit to be approximately the equatorial radius of Mars, we findthat Bottomos' period would be about 100 minutes . The Martian villagers wouldhardly need a calendar to keep track of Bottomos' arrival.
Having been reminded of puns by thinking about The Holes Around Mars , I shallconclude this section by describing Larry Niven's very short (fewer than 200 words)story Unfinished Story #1 (1970), which is essentially nothing but a pun (but, still, thestory contains some good physics) . A certain warlock keeps his cave cool in the summer by using a demon who sits at the entrance and keeps the fast-moving moleculesof air from entering and the slow-moving ones from leaving; the rest are allowed topass. Naturally, the process is reversed in the winter. When a visiting sorcerer compliments the warlock on his ingenuity, the warlock is quick to place the credit where itbelongs . The idea came from his clerk, Maxwell .'
PHYSICS AND SCIENCE FICTION
4. SCIENCE FICTION AS A 1EACHING mOL
553
My experience in teaching our department's course in Physics and Science Fictionhas shown me that science fiction can be used as an effective tool to teach the basicideas of both classical and modem physics (I also included some basic ideas ofastronomy and cosmology) to students with little background in science and mathematics . For the most part, the students seemed to learn a good bit and to enjoy doingso, and I found the course enjoyable to teach.
The science fiction readings , both short stories and novels, were chosen primarilybecause of their use (or misuse) ofphysics , although I certainly tried to choose storiesthat the students would enjoy reading . My lectures were about physics and includedmany demonstrations to illustrate the ideas, but I would also illustrate them by discussing how they fit into the assigned science fiction readings . I used UnfinishedStory #1, for example , to illustrate the concept that the Kelvin temperature of a gas isproportional to the mean kinetic energy of the random translational motions of themolecules (and I explained the story's pun) . The examinations tested their knowledgeofphysics, but I often worded the questions in ways that referred to the stories ; doingso provided added motivation for the students to actually do the readings withoutactually testing them on things unrelated to physics.
Although the emphasis was on understanding the ideas, the course was not whollyqualitative . For example, the students had to understand quantitatively what it meansfor something to be proportional to the inverse square of the distance, and they had touse conservation laws quantitatively to find how one thing changes given the changeof something else. They could calculate the gravitational acceleration on the surfaceof another planet knowing the factors by which its mass and radius differed from theEarth's, and by what factor the rotating men in The Men and the Mirror wouldincrease their angular velocity given the factor by which they decreased their momentof inertia as they pulled in on the rope joining them. They could also use graphs ofgravitational potential energy as a function of altitude to predict (quantitatively) thebehavior of objects launched from various altitudes with various kinetic energies .Still, the main emphasis was on having the students achieve a clear, qualitative understanding of the basic concepts .
Perhaps some readers connected with academic institutions feel motivated to tryteaching a course of this nature themselves . If so, and if they would like more information about the course that I taught, they should feel free to contact me.
University ofPittsburgh
554 A LLEN 1. JA NIS
NOTES
I. A more accurate calculation would use the equation of geodesic deviation, and would require knowingthe star's angular momentum and further details of the ship 's trajectory. Most neutron stars are rotat ing. Niven indicates that the one in the story does rotate, but he doesn 't give any indicat ion of how rapidly. An estimate using the equation of geodesic deviation , neglecting rotation and estimating therelevant trajecto ry data , indicates that tidal forces would still destroy Schae ffer.
2. For the sake of any reader unfamiliar with both Maxwell and his demon , let me point out that the fullname of the person whose idea Niven borrowed was James Clerk Maxwell , and that he is sometimesreferred to as Clerk Maxwell.
REFERENCES
Asimov, Isaac. 1952. The Martian Way. Reprint ed 1973 in The Best ofIsaac Asimov, Isaac Asimov.- - - . ed. 197I . Where Do We Go From Here? Greenwich, Connecticut : Fawcett .---. 1973. The Best ofIsaac Asimov. New York: Fawcett Crest.- - -. ed. 1974 . Bef ore the Golden Age. Garden City, New York: Doubl eday.Bixby, Jerome. 1954. The Holes Around Mars. Reprinted 1971 in Where Do We Go From Here? ed. Isaac
Asimov .Brin, D. 1983. "Tank-Farm Dynam o." Analog: Science Fiction/Science Fact. New York: Davis Publica -
tions, (November):104- I 18.Clarke , A. 1973. Rendezvous with Rama. New York: Ballantine.Hoyle , F. 1957. The Black Cloud . New York: New American Library.Janis, A. 1989. "Astronomy and Science Fiction." Pp. 233-249 in Cosmic Perspectives, ed. S.K. Biswas ,
D.C.V. Mall ik, and C.v. Vishveshwara. Cambridge: Cambridge University Press.Landis , G. 1999. "Tethered Satellites as 'Voodoo Science ' ." Physics & Society 28/1 :2, 14.Niven, Larry. 1966. Neutron Star. Reprinted 1971 in Where Do We Go From Here? ed. I. Asim ov.---. 1970. Unfinished StOlY #1. Reprinted 1971 in All the Myriad Ways, L. Niven.---. 1971. All the Myriad Ways, New York: Ballantine .Park , R. 1998. "Voodoo Science: Perpetuum Mobile." Physics & Society 27/4:4-5.Rocklynne, Ross . 1938. The Men and the Mirror: Reprint ed 1974 in Be/ ore the Golden Age, ed. Isaac Asi
