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RECENT EXPERIMENTS WITH INVISIBLE LIGHT.1

[With 6 plates.]

By R. W. WOOD, LL. D.,

Professor of Experimental Physics, Johns Hopkins University.

By far the greater proportion of the discoveries which have been made in natural science up to the present time depend upon observations made with the eye, either with or without the aid of optical instruments. The eye is, however, sensitive to only a very small part of the total radiation which reaches it, and it seems not unlikely that, if its range could be extended, many new phenomena would immediately come to light. By the employment of photography and of instruments which detect and measure the intensity of the infrared or heat rays, much new information has been gathered, especially in the science of spectroscopy; but usually these methods have been applied only in cases where the invisible radiations were known to be present. It seemed quite probable that if photographic methods were applied to various physical phenomena which excluded the action of any but invisible rays, new facts would probably be discovered. I can illustrate what I mean by taking two striking cases which were found at the very outset of the investigation, and which will be more fully discussed presently.

If the finger be dipped into powdered zinc oxide and rubbed over a sheet of white paper, eye observation is absolutely unable to detect the presence of the streaks made by the white powder, unless it has been very thickly applied. If, however, we photograph the paper with ultra-violet light we obtain a picture in which the streaks are as black as if made with powdered charcoal. This suggests that if we apply the process to the photography of the moon and planets, we have some reason to suspect that substances which can not be detected visually may come out in the photographs, a surmise which has been justified in one case at least. This and other similar cases will be taken up in detail presently.

As an illustration of how the method may be applied to the investigation of various physical phenomena, we may take another interesting case, in which a new radiant emission from the electric spark has been discovered. It was suspected that the very short

1 Lecture before the Royal Institution of Great Britain, Friday, May 19, 1911. Reprinted by permission from author's separate of Proceedings of the Royal Institution.

waves discovered by Schumann, which are powerfully absorbed by air, might possibly render the air fluorescent, the emitted light being invisible, however, on account of its short wave length. A heavy spark discharge was accordingly placed behind a small disk of metal, which cut off all the direct light, and the surrounding region photographed with a quartz lens, which is transparent to the ultra-violet rays. It was found that the air in the neighborhood of the spark actually did give off actinic invisible rays, the photograph giving the impression of a luminous fog surrounding the metal disk.

I will now show you an experiment which illustrates that two objects which can not be distinguished under ordinary illumination may appear quite different when the light which illuminates them is restricted to certain regions of the spectrum. I have here two pieces of scarlet silk which can not be distinguished the one from the other in the light of the incandescent electric lamps which illuminate this room. I now extinguish the lamps and place the two pieces of silk under this Cooper-Hewitt mercury arc lamp, and as you see, one of them still appears scarlet as before, while the other appears very dark blue, almost black, in fact. The peculiarity of the mercury lamp lies in the fact that it gives out little or no red light, consequently red objects in general appear almost black. The peculiarity of this particular piece of silk, by virtue of which it appears quite as red as in ordinary lights, lies in the fact that the red dye with which it is colored is fluorescent under the action of the green rays from the lamp; the red light is manufactured, so to speak, from the green light by the coloring matter of the silk. If I place the arc lamp and the piece of silk behind this large sheet of red glass, you will observe that the fabric is actually brighter than the lamp itself, probably eight or ten times as bright. We can form an image of the lamp on the silk with a lens, and the image will be many times brighter than the lamp, which might be taken as a refutation of the old and well-known theorem in optics that no optical system can yield an image brighter than the source (!) Here is another piece of white silk upon which I have made some red spots with this same dye. By the ordinary illumination of the room it is seen to be white, with large pink polka dots, something quite suitable for a young lady's summer gown. I now place it behind the red screen under the mercury arc and it at once becomes quite diabolical in appearance, bluish-black with flaming spots of scarlet, entirely unsuitable for the aforementioned purpose. The dye which was used for coloring these fluorescent fabrics was rhodamin. The conditions of illumination and observation are, of course, rather special in these cases, I have introduced them merely to illustrate how the eye may be deceived under certain conditions.

and

Practically all sources of light in ordinary use give out more or less ultra-violet light which plays no part in vision, but which can be rendered apparent in various ways. I have on the table a new arrangement by which these rays can be separated from the visible ones. The apparatus is practically identical with the device quite recently used by Prof. Rubens and myself for isolating the longest heat waves that have been discovered up to the present time. It can be used as well for the isolation of the ultra-violet, since its action depends upon the high refractive index which quartz has for these two types of radiation. The source is, in this case, an electric spark contained in this box, and the ultra-violet rays are brought to a focus upon a small circular aperture in a cardboard screen. The focal length of the lens is so much greater for visible light that these rays do not come to a focus at all, but are spread over a circular area of a diameter nearly half that of the lens.

A penny has been fastened to the center of the lens with wax, and this shields the aperture from the cone of visible rays coming from the central portions of the lens. If I hold a sheet of white paper above the aperture you observe that it remains dark-that is, no visible rays pass through to the paper; if, however, I substitute for the paper this mass of uranium nitrate crystals, the presence of the ultra-violet rays is made manifest, the crystals shining with a brilliant green light.

Certain vapors shine with a brilliant light when exposed to these invisible rays. One of the most striking is the vapor of metallic mercury, which I can show you by boiling the metal in this flask of fused quartz placed above the aperture. The metal is boiling now, and you can all see the brilliant cone of green light which marks the path of the ultra-violet rays through the metallic vapor. If I hold a thin sheet of glass between the aperture and the flask, you will observe that the vapor instantly becomes dark, for the glass stops completely the rays in question.

The vapor of mercury exhibits an absorption band in the ultraviolet region which resembles the band at wave-length 5893 shown by dense sodium vapor. So powerful is this absorption that I have detected it in the vapor of mercury at room temperature. It occurred to me that this light instead of being absorbed might possibly be reemitted by the vapor laterally in all directions. To test this point I sealed up a drop of mercury in an exhausted flask of quartz, and focused the light of the mercury arc (burning in a silica tube) at the center of the bulb, which was not heated. The bulb was then photographed with a quartz lens, and the picture clearly showed the cone of focused rays precisely as if the bulb were filled with smoke. This is another very good example of how new discoveries may be made by ultra-violet photography.

If the object to be photographed gives off visible rays in addition to the invisible ones, it is necessary to remove these by a suitable screen or ray filter. We will begin by considering some remarkable effects which are obtained when sunlit landscapes are photographed by means of the obscure rays at the extreme red end of the spectrum. A screen can be prepared which transmits these rays, and is at the same time opaque to all other radiations, by combining a sheet of the densest blue cobalt glass with a solution of bichromate of potash or some suitable orange dye.

Such a screen transmits a region of the spectrum comprised between wave lengths 6900 and 7500. Though this region is visible to the eye if all other rays are cut off, it is so feeble in its action that it plays no part in ordinary vision, being overpowered by the other radiations. We may thence, for convenience, call photographs made through such a screen infra-red pictures, though the infra-red region is usually considered as beginning at the point where all action upon the human retina ceases.

The photographs which I am now going to show you were taken through such a screen, with the spectrum plates made by Wratten and Wainwright. The time of exposure was about three minutes in full sunlight, with the lens stop set at f/8. The views were, for the most part, made in Sicily and Italy, and have a very curious appearance, for while the sky comes out in all of them almost as black as midnight, the foliage of the trees and the grass come out snow white. This peculiar effect results from the failure of the atmosphere to scatter these long rays. The green leaves, however, reflect them very powerfully, or, more correctly, transmit them, since we are dealing with pigment or transmission color. If we look at a landscape through the screen, carefully protecting the eye from all extraneous light with a black cloth, we shall find that the trees shine with a beautiful rich red light against a black sky. This condition obtains only on very clear days, for the presence of the least haze in the air enables it to scatter the long rays, and you will notice that in those pictures which show the sky down to the horizon there is a progressive increase in its luminosity as we pass from the zenith downward, as a result of the greater thickness of the mass of air sending the scattered rays to the camera.

Another point to be noticed is the intense blackness of the shadows. in the infra-red pictures, due to the fact that most of the light comes directly from the sun and little or none from the sky, which reminds one forcibly of the conditions which obtain on the moon, where there is no atmosphere at all to form a luminous sky.

When we come to the subject of photographs made with ultraviolet light, we shall find that we have the conditions reversed, for

practically all of these very short waves are scattered by the atmosphere, and we have no shadows even in full sunlight.

We will now run through the series of infra-red pictures as rapidly as possible, for I have a considerable number of them. The one which is on the screen is one of the finest in the collection (pl. 1). It was made in the park at Florence, and shows the long drive, overshadowed by trees, the one in the foreground being particularly fine in appearance. The next one (pl. 2) was made at the bottom of one of the old quarries or latomiæ at Syracuse, the view looking out through a cavelike formation at a group of almond trees, with which the quarry is overgrown.

Here is a fine row of cypresses growing by an old gate, taken on a somewhat hazy day, with the sky appearing a little lighter than usual. Some of the pictures show the advantage gained in bringing out the detail of distant objects seen through the atmospheric haze, and it does not seem impossible that photographs of the brighter planets made through an infra-red screen might prove interesting if the planets are surrounded by a light scattering atmosphere, for we must bear in mind that the surface of the earth, as seen from a neighboring planet, would be seen through a luminous haze, equal in brilliance to the blue sky on a clear day; that is, it would present much the same appearance as is presented by the moon when seen at noonday.

We will now look into the question of how things would appear if our eyes were sensitive only to ultra-violet light. In applying the same method which we have used for the infra-red, we require a screen which is opaque to all visible light, but which transmits the ultra-violet.

Glass is opaque to these rays, cutting them off almost completely, and for this reason we can not employ glass lenses. Quartz, on the other hand, is exceedingly transparent to these invisible rays, but it is a little difficult to find a medium which is transparent to them and at the same time quite opaque to visible light. Indeed, there is only one substance known which completely fulfills such a condition, namely, metallic silver. If we deposit chemically a thin film of metallic silver on the surface of a quartz lens, a certain amount of ultra-violet radiation between 3000 and 3200 is able to struggle through and form an image on the plate.

I have used silver films through which the filament of a tungsten. lamp is invisible. The best thickness is that at which the tungsten. lamp is just barely discernible. If the objects to be photographed are illuminated with the light of an electric spark, or some other source, rich in ultra-violet rays, much thinner films of silver can be employed, but in the case of sunlight, which has passed through the earth's atmosphere, the ultra-violet in the region for which silver has

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