Part 32

E. C. Slipher's paper in Popular Astronomy for March, 1914, gives a good account of the recent work.

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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

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

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?"

De Quincy in his System of the Heavens gives a fine paraphrase of "Richter's Dream."

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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."

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THE LATENT IMAGE

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

Presidential address to the Photographic Convention of the United Kingdom, July, 1905. Nature, Vol. 72, p. 308.

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chemical activity at the comparatively elevated temperature of -180 deg. 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

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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

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