September 27, 1889.] 



SCIENCE. 



22P 



rays to become photographic rays. Some eight years ago it was 

 my own good fortune to make the dark infra-red rays impress 

 themselves on a plate. This last has been too much a specialty of 

 my own, although full explanations have been given of the methods 

 employed. By preparing a bromide-of-silver salt in a peculiar 

 manner, one is able, so to modify the molecular arrangement of the 

 atoms that they answer to the swings of those waves which give 

 rise to these radiations. By employing this salt of silver in a film 

 of collodion or gelatine, the invisible part of the spectrum can be 

 photographed, and the images of bodies which are heated to less 

 than red heat may be caused to impress themselves upon the sen- 

 sitive plate. The greatest wave-length of the spectrum to which 

 this salt is sensitive, so far, is 22,000 X, or five times the length of 

 the visible spectrum. The exposure for such a wave-length is very 

 prolonged; but down to a wave-length of 12,000 it is compara- 

 tively short, though not so short as that required for the blue rays 

 to impress themselves on a collodion plate. The color of the sen- 

 sitive salt is a green-blue by transmitted light. It has yet to be de- 

 termined whether this color is all due to the coarseness of the par- 

 ticles, or to the absorption by the molecules. The fact that a film 

 can be prepared which by transmitted light is yellow, and which 

 may be indicative of color due to fine particles, together with an 

 absorption of the red and orange, points to the green color being 

 probably due to absorption by the molecules. We have thus in 

 photography a means of recording phenomena in the spectrum 

 from the ultra-violet to a very large wave-length in the infra-red, — 

 a power which physicists may some day turn to account. It 

 would, for instance, be a research worth pursuing to photograph 

 the heavens on a plate prepared with such a salt, and search for 

 stars which are nearly dead or newly born ; for in both cases the 

 temperature at which they are may be such as to render them be- 

 low red-heat, and therefore invisible to the eye in the telescope. 

 It would be a supplementary work to that being carried out by the 

 brothers Henri, Common, Roberts, Gill, and others, who are busy 

 securing photographic charts of the heavens in a manner which is 

 beyond praise. 



There is one other recent advance which has been made in sci- 

 entific photography to which I may be permitted to allude ; viz., 

 that, from being merely a qualitative recorder of the action of light, 

 it can now be used for quantitative measurement. I am not now 

 alluding to photographic actinometers, such as have been brought 

 to such a state of perfection by Roscoe, but what I allude to is the 

 measurement and interpretation of the density of deposit in a nega- 

 tive. By making exposures of different lengths to a standard light, 

 or to different known intensities of light, on the same plate on 

 which a negative has to be taken, the photographic values of the 

 light acting to produce the densities on the different parts of the 

 developed image can be readily found. Indeed, by making only 

 two different exposures to the same light, or two exposures to two 

 different intensities of light, and applying the law of density of de- 

 posit in regard to them, a curve is readily made from which the in- 

 tensities of light necessary to give the different densities of deposit 

 in the image impressed on the same plate can be read off. The ap- 

 plication of such scales of density to astronomicar photographs, for 

 example, cannot but be of the highest interest, and will render the 

 records so made many times more valuable than they have hith- 

 erto been. I am informed that the United States astronomers 

 have already adopted the use of such scales, which for the last 

 three years I have advocated, and it may be expected that we shall 

 have results from such scaled photographs which will give us in- 

 formation which would before have been scarcely hoped for. 



One word as to a problem which we may say is as yet only 

 qualitatively and not quantitatively solved. I refer to the inter- 

 changeability of length of exposure for intensity of light. Put it in 

 this way. Suppose that with a strong light, L, a. short exposure, 

 E, being given, a chemical change, C, is obtained : will the same 

 change, C, be obtained if the time is only an «th of the light, L, 

 but n times the exposure .' Now, this is a very important point, 

 more particularly when the body acted upon is fairly stable ; as, 

 for instance, some of the water-color pigments, which are known 

 to fade in sunshine, but might not be supposed to do so in the 

 light of an ordinary room, even with prolonged exposure. Many 

 experiments have been made at South Kensington as regards this. 



more especially with the salts of silver ; and it is found, that, for 

 any ordinary light, intensity and exposure are interchangeable, but 

 that when the intensity of light is very feeble, say the Tmroinjtr of 

 ordinary daylight, the exposure has to be rather more prolonged 

 than it should be supposing the exact interchangeability always 

 held good. But it has never been found that a light was so feeble 

 that no action could take place. Of course, it must be borne in 

 mind that the stability of the substance acted upon may have 

 some effect ; but the same results were obtained with matter 

 which is vastly more stable than the ordinary silver salts. It may 

 be said, in truth, that almost all matter which is not elemental is- 

 in time, and to some degree, acted upon by light. 



I should like to have said something regarding the action of hght 

 on the iron and chromium salts, and so introduced the subject of 

 platinotype and carbon printing, the former of which is creating a.! 

 revolution in the production of artistic prints. I have, however, re- 

 frained from so doing, as I felt that the president of Section A. 

 should, not be mistaken as the president of Section B. Photogra- 

 vure and the kindred processes were also inviting subjects on 

 which to dwell, more especially as at least one of them is based on 

 the use of the same material as that on which the first camera pic- 

 ture was taken by Niepce. Again, a dread of trenching on the do- 

 mains of art restrains me. 



Indeed, it would have been almost impossible, and certainly im- 

 politic, in the time which an address should occupy, to have en- 

 tered into the many branches of science and art which photography 

 covers. I have tried to confine myself to some few advances that 

 have been made in its theory and practice. 



The discovery of the action of light on silver salts is one of the- 

 marvels of this century, and it is difficult to overrate the bearing it 

 has had on the progress of science, more especially physical sci- 

 ence. The discovery of telegraphy took place in the present reign,, 

 and two yfers later photography was practically introduced ; and 

 no two discoveries have had a more marked influence on mankind- 

 Telegraphy, however, has had an advantage over photography in 

 the scientific progress that it has made, in that electrical currents 

 are subject to exact measurement, and that empiricism has no 

 place with it. Photography, on the other hand, has labored under 

 the disadvantage, that, though it is subject to measurement, the 

 factors of exactitude have been hitherto absent. In photography 

 we have to deal with molecules the equiUbrium of whose compo- 

 nents is more or less indifferent according to the process used- 

 Again, the light employed is such a varying factor that it is diffi- 

 cult to compare results. Perhaps more than any other disadvan- 

 tage it labors under, is that due to quackery of the worst descrip- 

 tion at the hands of some of its followers, who not only are self- 

 asserting, but often ignorant of the very first principles of scientific 

 investigation. Photography deserves to have followers of the high- 

 est scientific calibre ; and, if only some few more real physicists 

 and chemists could be induced to unbend their minds and study 

 the theory of an applied science which they often use for record or 

 for pleasure, we might hope for some greater advance than has 

 hitherto been possible. 



Photography has been called the handmaid of art : I venture to 

 think it is even more so the handmaid of science, and each step- 

 taken in perfecting it will render it more worthy of such a title. 



ELECTRICAL NEWS. 

 Recent Fatalities from Electricity. 

 One death and several serious injuries from electric-light wires 

 have occurred during the past two weeks. Some days ago the 

 eight-year-old son of Charles Kern of Baltimore came in contact 

 with an electric-light wire while looking out of a window, lost his 

 balance, and fell to the street. A New York daily newspaper, al- 

 luding to the fact, stated that the boy was " fairly lifted out of the 

 room, and hurled into the street ; " all of which is interesting, if 

 true. John Powers, an employee of The Brush Electric-Light 

 Company, thoughtlessly took hold of a live wire with one hand, 

 and with the other made an excellent ground connection with the 

 Elevated Railroad structure on East 34th Street. He was standing 

 on a step-ladder at the time ; and the shock of the fall, not the cur- 

 rent, killed him. Some days after this occurrence a poor vagrant,. 



