Chapter 13

In the first place the curves to which I have but briefly referred actually give rise in most cases to nodal, or crossing points; sometimes on the equator, sometimes off the equator; through which the path of the satellite returns again and again. These nodal points will not, however, afford a general explanation of the many-branched radiants.

It is probable that we should refer such an appearance

193

as is shown at the Sinus Titanum to the perturbations of the satellite's path due to the surface features on Mars. Observe that the principal radiants are situated upon the boundary of the dark regions or at the oases. Higher surface levels may be involved in both cases. Some marked difference in topography must characterise both these features. The latter may possibly originate in the destruction of satellites. Or again, they may arise in crustal disturbance of a volcanic nature, primarily induced or localised by the crossing of two canals. Whatever the origin of these features it is only necessary to assume that they represent elevated features of some magnitude to explain the multiplication of crossing lines. We must here recall what observers say of the multiplicity of the canals. According to Lowell, "What their number maybe lies quite beyond the possibility of count at present; for the better our own air, the more of them are visible."

Such innumerable canals are just what the present theory requires. An in-falling satellite will, in the course of the last 60 or 80 years of its career, circulate some 100,000 times over Mars' surface. Now what will determine the more conspicuous development of a particular canal? The mass of the satellite; the state of the surface crust; the proximity of the satellite; and the amount of repetition over the same ground. The after effects may be taken as proportional to the primary disturbance.

194

It is probable that elevated surface features will influence two of these conditions: the number of repetitions and the proximity to the surface. A tract 100 miles in diameter and elevated 5,000 or 10,000 feet would seriously perturb the orbit of such a body as Phobos. It is to be expected that not only would it be effective in swaying the orbit of the satellite in the horizontal direction but also would draw it down closer to the surface. It is even to be considered if such a mass might not become nodal to the satellite's orbit, so that this passed through or above this point at various inclinations with its primary direction. If acting to bring down the orbit then this will quicken the speed and cause the satellite further on its path to attain a somewhat higher elevation above the surface. The lines most conspicuous in the telescope are, in short, those which have been favoured by a combination of circumstances as reviewed above, among which crustal features have, in some cases, played a part.

I must briefly refer to what is one of the most interesting features of the Martian lines: the manner in which they appear to come and go like visions.

Something going on in Mars determines the phenomenon. On a particular night a certain line looks single. A few nights later signs of doubling are perceived, and later still, when the seeing is particularly good, not one but two lines are seen. Thus, as an example, we may take the case of Phison and Euphrates. Faint glimpses of the dual state were detected in the summer

195

and autumn, but not till November did they appear as distinctly double. Observe that by this time the Antarctic snows had melted, and there was in addition, sufficient time for the moisture so liberated to become diffused in the planet's atmosphere.

This increase in the definition and conspicuousness of certain details on Mars' surface is further brought into connection with the liberation of the polar snows and the diffusion of this water through the atmosphere, by the fact that the definition appeared progressively better from the south pole upwards as the snow disappeared. Lowell thinks this points to vegetation springing up under the influence of moisture; he considers, however, as we have seen, that the canals convey the moisture. He has to assume the construction of triple canals to explain the doubling of the lines.

If we once admit the canals to be elevated ranges--not necessarily of great height--the difficulty of accounting for increased definition with increase of moisture vanishes. We need not necessarily even suppose vegetation concerned. With respect to this last possibility we may remark that the colour observations, upon which the idea of vegetation is based, are likely to be uncertain owing to possible fatigue effects where a dark object is seen against a reddish background.

However this may be we have to consider what the effects of moisture increasing in the atmosphere of Mars will be with regard to the visibility of elevated ranges,

196

We assume a serene and rare atmosphere: the nights intensely cold, the days hot with the unveiled solar radiation. On the hill tops the cold of night will be still more intense and so, also, will the solar radiation by day. The result of this state of things will be that the moisture will be precipitated mainly on the mountains during the cold of night--in the form of frost--and during the day this covering of frost will melt; and, just as we see a heavy dew-fall darken the ground in summer, so the melting ice will set off the elevated land against the arid plains below. Our valleys are more moist than our mountains only because our moisture is so abundant that it drains off the mountains into the valleys. If moisture was scarce it would distil from the plains to the colder elevations of the hills. On this view the accentuation of a canal is the result of meteorological effects such as would arise in the Martian climate; effects which must be influenced by conditions of mountain elevation, atmospheric currents, etc. We, thus, follow Lowell in ascribing the accentuation of the canals to the circulation of water in Mars; but we assume a simple and natural mode of conveyance and do not postulate artificial structures of all but impossible magnitude. That vegetation may take part in the darkening of the elevated tracts is not improbable. Indeed we would expect that in the Martian climate these tracts would be the only fertile parts of the surface.

Clouds also there certainly are. More recent observations

197

appear to have set this beyond doubt. Their presence obviously brings in other possible explanations of the coming and going of elevated surface features.

Finally, we may ask what about the reliability of the maps? About this it is to be said that the most recent map--that by Lowell--has been confirmed by numerous drawings by different observers, and that it is,itself the result of over 900 drawings. It has become a standard chart of Mars, and while it would be rash to contend for absence of errors it appears certain that the trend of the principal canals may be relied on, as, also, the general features of the planet's surface.

The question of the possibility of illusion has frequently been raised. What I have said above to a great extent answers such objections. The close agreement between the drawings of different observers ought really to set the matter at rest. Recently, however, photography has left no further room for scepticism. First photographed in 1905, the planet has since been photographed many thousands of times from various observatories. A majority of the canals have been so mapped. The doubling of the canals is stated to have been also so recorded.[1]

The hypothesis which I have ventured to put before you involves no organic intervention to account for the

[1] E. C. Slipher's paper in _Popular Astronomy_ for March, 1914, gives a good account of the recent work.

198

details on Mars' surface. They are physical surface features. Mars presents his history written upon his face in the scars of former encounters--like the shield of Sir Launcelot. Some of the most interesting inferences of mathematical and physical astronomy find a confirmation in his history. The slowing down in the rate of axial rotation of the primary; the final inevitable destruction of the satellite; the existence in the past of a far larger number of asteroids than we at present are acquainted with; all these great facts are involved in the theory now advanced. If justifiably, then is Mars' face a veritable Principia.

To fully answer the question which heads these lectures, we should go out into the populous solitudes (if the term be permitted) which lie beyond our system. It is well that there is now no time left to do so; for, in fact, there we can only dream dreams wherein the limits of the possible and the impossible become lost.

The marvel of the infinite number of stars is not so marvellous as the rationality that fain would comprehend them. In seeking other minds than ours we seek for what is almost infinitely complex and coordinated in a material universe relatively simple and heterogeneous. In our mental attitude towards the great question, this fact must be regarded as fundamental.

I can only fitly close a discourse which has throughout weighed the question of the living thought against the unthinking laws of matter, by a paraphrase of the words

199

of a great poet when he, in higher and, perhaps, more philosophic language, also sought to place the one in comparison with the other.[1]

Richter thought that he was--with his human heart unstrengthened--taken by an angel among the universe of stars. Then, as they journeyed, our solar system was sunken like a faint star in the abyss, and they travelled yet further, on the wings of thought, through mightier systems: through all the countless numbers of our galaxy. But at length these also were left behind, and faded like a mist into the past. But this was not all. The dawn of other galaxies appeared in the void. Stars more countless still with insufferable light emerged. And these also were passed. And so they went through galaxies without number till at length they stood in the great Cathedral of the Universe. Endless were the starry aisles; endless the starry columns; infinite the arches and the architraves of stars. And the poet saw the mighty galaxies as steps descending to infinity, and as steps going up to infinity.

Then his human heart fainted and he longed for some narrow cell; longed to lie down in the grave that he might hide from infinity. And he said to the angel:

"Angel, I can go with thee no farther. Is there, then, no end to the universe of stars?"

[1] De Quincy in his _System of the Heavens_ gives a fine paraphrase of "Richter's Dream."

200

Then the angel flung up his glorious hands to the heaven of heavens, saying "End is there none to the universe of God? Lo! also there is no beginning."

201

THE LATENT IMAGE [1]

My inclination has led me, in spite of a lively dread of incurring a charge of presumption, to address you principally on that profound and most subtle question, the nature and mode of formation of the photographic image. I am impelled to do so, not only because the subject is full of fascination and hopefulness, but because the wide topics of photographic methods or photographic applications would be quite unfittingly handled by the president you have chosen.

I would first direct your attention to Sir James Dewar's remarkable result that the photographic plate retains considerable power of forming the latent image at temperatures approaching the absolute zero--a result which, as I submit, compels us to regard the fundamental effects progressing in the film under the stimulus of light undulations as other than those of a purely chemical nature. But few, if any, instances of chemical combination or decomposition are known at so low a temperature. Purely chemical actions cease, indeed, at far higher temperatures, fluorine being among the few bodies which still show

[1] Presidential address to the Photographic Convention of the United Kingdom, July, 1905. _Nature_, Vol. 72, p. 308.

202

chemical activity at the comparatively elevated temperature of -180° C. In short, this result of Sir James Dewar's suggests that we must seek for the foundations of photographic action in some physical or intra-atomic effect which, as in the case of radioactivity or fluorescence, is not restricted to intervals of temperature over which active molecular vis viva prevails. It compels us to regard with doubt the role of oxidation or other chemical action as essential, but rather points to the view that such effects must be secondary or subsidiary. We feel, in a word, that we must turn for guidance to some purely photo-physical effect.

Here, in the first place, we naturally recall the views of Bose. This physicist would refer the formation of the image to a strain of the bromide of silver molecule under the electric force in the light wave, converting it into what might be regarded as an allotropic modification of the normal bromide which subsequently responds specially to the attack of the developer. The function of the sensitiser, according to this view, is to retard the recovery from strain. Bose obtained many suggestive parallels between the strain phenomena he was able to observe in silver and other substances under electromagnetic radiation and the behaviour of the photographic plate when subjected to long-continued exposure to light.

This theory, whatever it may have to recommend it, can hardly be regarded as offering a fundamental explanation. In the first place, we are left in the dark as to what

203

the strain may be. It may mean many and various things. We know nothing as to the inner mechanism of its effects upon subsequent chemical actions--or at least we cannot correlate it with what is known of the physics of chemical activity. Finally, as will be seen later, it is hardly adequate to account for the varying degrees of stability which may apparently characterise the latent image. Still, there is much in Bose's work deserving of careful consideration. He has by no means exhausted the line of investigation he has originated.

Another theory has doubtless been in the minds of many. I have said we must seek guidance in some photo-physical phenomenon. There is one such which preeminently connects light and chemical phenomena through the intermediary of the effects of the former upon a component part of the atom. I refer to the phenomena of photo-electricity.

It was ascertained by Hertz and his immediate successors that light has a remarkable power of discharging negative electrification from the surface of bodies--especially from certain substances. For long no explanation of the cause of this appeared. But the electron--the ubiquitous electron--is now known with considerable certainty to be responsible. The effect of the electric force in the light wave is to direct or assist the electrons contained in the substance to escape from the surface of the body. Each electron carries away a very small charge of negative electrification. If, then, a body is

204

originally charged negatively, it will be gradually discharged by this convective process. If it is not charged to start with, the electrons will still be liberated at the surface of the body, and this will acquire a positive charge. If the body is positively charged at first, we cannot discharge it by illumination.

It would be superfluous for me to speak here of the nature of electrons or of the various modes in which their presence may be detected. Suffice it to say, in further connection with the Hertz effect, that when projected among gaseous molecules the electron soon attaches itself to one of these. In other words, it ionises a molecule of the gas or confers its electric charge upon it. The gaseous molecule may even be itself disrupted by impact of the electron, if this is moving fast enough, and left bereft of an electron.

We must note that such ionisation may be regarded as conferring potential chemical properties upon the molecules of the gas and upon the substance whence the electrons are derived. Similar ionisation under electric forces enters, as we now believe, into all the chemical effects progressing in the galvanic cell, and, indeed, generally in ionised solutes.

An experiment will best illustrate the principles I wish to remind you of. A clean aluminium plate, carefully insulated by a sulphur support, is faced by a sheet of copper-wire-gauze placed a couple of centimetres away from it. The gauze is maintained at a high positive

205

potential by this dry pile. A sensitive gold-leaf electroscope is attached to the aluminium plate, and its image thrown upon the screen. I now turn the light from this arc lamp upon the wire gauze, through which it in part passes and shines upon the aluminium plate. The electroscope at once charges up rapidly. There is a liberation of negative electrons at the surface of the aluminium; these, under the attraction of the positive body, are rapidly removed as ions, and the electroscope charges up positively.

Again, if I simply electrify negatively this aluminium plate so that the leaves of the attached electroscope diverge widely, and now expose it to the rays from the arc lamp, the charge, as you see, is very rapidly dissipated. With positive electrification of the aluminium there is no effect attendant on the illumination.

Thus from the work of Hertz and his successors we know that light, and more particularly what we call actinic light, is an effective means of setting free electrons from certain substances. In short, our photographic agent, light, has the power of expelling from certain substances the electron which is so potent a factor in most, if not in all, chemical effects. I have not time here to refer to the work of Elster and Geitel whereby they have shown that this action is to be traced to the electric force in the light wave, but must turn to the probable bearing of this phenomenon on the familiar facts of photography. I assume that the experiment I have shown you is the most

206

fundamental photographic experiment which it is now in our power to make.

We must first ask from what substances can light liberate electrons. There are many--metals as well as non-metals and liquids. It is a very general phenomenon and must operate widely throughout nature. But what chiefly concerns the present consideration is the fact that the haloid salts of silver are vigorously photo-electric, and, it is suggestive, possess, according to Schmidt, an activity in the descending order bromide, chloride, iodide. This is, in other words, their order of activity as ionisers (under the proper conditions) when exposed to ultra-violet light. Photographers will recognise that this is also the order of their photographic sensitiveness.

Another class of bodies also concerns our subject: the special sensitisers used by the photographer to modify the spectral distribution of sensibility of the haloid salts, _e.g._ eosine, fuchsine, cyanine. These again are electron-producers under light stimulus. Now it has been shown by Stoletow, Hallwachs, and Elster and Geitel that there is an intimate connection between photo-electric activity and the absorption of light by the substance, and, indeed, that the particular wave-lengths absorbed by the substance are those which are effective in liberating the electrons. Thus we have strong reason for believing that the vigorous photo-electric activity displayed by the special sensitisers must be dependent upon their colour absorption. You will recognise that this is just

207

the connection between their photographic effects and their behaviour towards light.

There is yet another suggestive parallel. I referred to the observation of Sir James Dewar as to the continued sensitiveness of the photographic film at the lowest attained extreme of temperature, and drew the inference that the fundamental photographic action must be of intra-atomic nature, and not dependent upon the vis viva of the molecule or atom. In then seeking the origin of photographic action in photo-electric phenomena we naturally ask, Are these latter phenomena also traceable at low temperatures? If they are, we are entitled to look upon this fact as a qualifying characteristic or as another link in the chain of evidence connecting photographic with photo-electric activity.

I have quite recently, with the aid of liquid air supplied to me from the laboratory of the Royal Dublin Society, tested the photo-sensibility of aluminium and also of silver bromide down to temperatures approaching that of the liquid air. The mode of observation is essentially that of Schmidt--what he terms his static method. The substance undergoing observation is, however, contained at the bottom of a thin copper tube, 5 cm. in diameter, which is immersed to a depth of about 10 cm in liquid air. The tube is closed above by a paraffin stopper which carries a thin quartz window as well as the sulphur tubes through which the connections pass. The air within is very carefully dried by phosphorus

208

pentoxide before the experiment. The arc light is used as source of illumination. It is found that a vigorous photo-electric effect continues in the case of the clean aluminium. In the case of the silver bromide a distinct photo-electric effect is still observed. I have not had leisure to make, as yet, any trustworthy estimate of the percentage effect at this temperature in the case of either substance. Nor have I determined the temperature accurately. The latter may be taken as roughly about -150° C,

Sir James Dewar's actual measilrements afforded twenty per cent. of the normal photographic effect at -180° C. and ten per cent. at the temperature of -252.5° C.

With this much to go upon, and the important additional fact that the electronic discharge--as from the X-ray tube or from radium--generates the latent image, I think we are fully entitled to suggest, as a legitimate lead to experiment, the hypothesis that the beginnings of photographic action involve an electronic discharge from the light-sensitive molecule; in other words that the latent image is built up of ionised atoms or molecules the result of the photo-electric effect on the illuminated silver haloid, and it is upon these ionised atoms that the chemical effects of the developer are subsequently directed. It may be that the liberated electrons ionise molecules not directly affected, or it may be that in their liberation they disrupt complex molecules built up in the ripening of the

209

emulsion. With the amount we have to go upon we cannot venture to particularise. It will be said that such an action must be in part of the nature of a chemical effect. This must be admitted, and, in so far as the rearrangement of molecular fabrics is involved, the result will doubtless be controlled by temperature conditions. The facts observed by Sir James Dewar support this. But there is involved a fundamental process--the liberation of the electron by the electric force in the light wave, which is a physical effect, and which, upon the hypothesis of its reality as a factor in forming the latent image, appears to explain completely the outstanding photographic sensitiveness of the film at temperatures far below those at which chemical actions in general cease.

Again, we may assume that the electron--producing power of the special sensitiser or dye for the particular ray it absorbs is responsible, or responsible in part, for the special sensitiveness it confers upon the film. Sir Wm. Abney has shown that these sensitisers are active even if laid on as a varnish on the sensitive surface and removed before development. It must be remembered, however, that at temperatures of about -50° these sensitisers lose much of their influence on the film; as I have pointed out in a paper read before the Photographic Convention of 1894.