own and in foreign countries, and it may be permitted to throw a glance from "the mind's eye" into the future and imagine an observatory in Great Britain which shall more than rival those of other countries. One can figure to oneself a tower piercing the sky from any of the elevated table-lands of this island, Salisbury Plain, the Stray at Harrogate, or anywhere on the downs between Guildford and Dorking, from which the most interesting results could not fail to accrue. It is the opinion of M. Vallot-no mean authority-that a high tower is for air-observing purposes equivalent to a mountain station of ten times the altitude, and this is evident when one considers that any mountain must act as an obstacle which thrusts the layers of the atmosphere upward into a contour almost like its own, so that some of the effects are very little different from those observed below. A tower like the Eiffel Tower, on the contrary, thrusts itself into the air without impeding its movements. Among the new subjects which might with advantage be studied from such an observatory are the systematic photography of the clouds all around the horizon and the effects of observed refraction in the different air strata, a subject only yet in its infancy; for Mr. H. F. Newall showed only last Friday to the Fellows of the Royal Astronomical Society how he had observed, in the great telescope of Cambridge, waves of a varying speed and frequency crossing each other at different angles in the field of view when the telescope was pointed at the open sky. He says these belong to the upper air 4 or 5 miles from the earth, and if he is right (which I hope), here alone is a new field of study which may be fruitful of results in the future.1 It is the boast of our society that it is covering the face of the country, and indeed of the world, with a network of private observing stations, and it is collecting together for the enlightenment of all future time a mass of accurate knowledge on the subject of the thousand changes in our atmosphere, its varying moods, its beating pulses, its calms and its convulsions, so that when the philosopher is born who is destined to unravel all its mysteries he will find the means and instruments made ready to his hand. The Observatory, 1896, p. 77. COLOR PHOTOGRAPHY BY MEANS OF BODY COLORS, AND By OTTO WIENER. I. SCOPE OF THE INVESTIGATION. In the investigation of fixed lightwaves I came at once upon the question of the fundamental possibility of color photography. Zenker had explained the processes then in use by the action of stationary lightwaves. Objections to the explanation not as yet overthrown are offered in an article by Schultz-Sellack. On this account, and because I was unacquainted with the possibility of the production of thicker transparent photographic films, I considered the solution of the question must be sought in other directions. These difficulties were, however, soon after overcome by Lippman," and he succeeded in obtaining a process of color photography by a suitable production of stationary lightwaves, and thus by the application of Zenker's theory. That this theory, however, explained the older processes was not yet proved, and I was unable to find anything looking to such a proof thoroughly established. I determined, therefore, to discover by new experiments the cause of the color production in the older procedures. These experiments form the point of departure and a considerable portion of the following communication. The objections of Schultz-Sellack are by no means to be brushed aside without further consideration. He disputed the fact of regular fixed lightwaves in powders. Powdered substances had been used for color production in the first process of Seebeck, whose observations were Translated from Annalen der Physik und Chemie, Neue Folge. Band 55. Leipzig. 1895. Wiener, Annalen der Physik und Chemie, 40: p. 205, 1890. Zenker, Lehrbuch der Photochromie, Berlin, private publication by the author, 1868. In my earlier investigation I found that Lord Rayleigh also, in connection with the investigation of wave propagation in a medium of periodic structure (Philosophical magazine (5) 24: p. 158, 1887), had considered the possibility of this explanation. He was, however, unacquainted with the theory of Zenker published nineteen years before. Schultz-Sellack, "Upon the coloration of turbid media and the so-called color photography." Annalen der Physik und Chemie, 143: p. 449, 1871. published in Goethe's "Farbenlehre," in the year 1810. Seebeck used moist chloride of silver which had become gray by the action of light, spreading this upon paper. Schultz-Sellack's objection applies with the greatest force to processes where paper is coated with the substance sensitive to light by soaking it in different solutions, as, for example, in Poitevin's process. It does not, on the other hand, apply in those which make use of a uniform transparent layer of a substance sensitive to light with a good reflecting background, as the processes of Becquerel, in which bright silver plates are chlorinized to a determined depth by electrolysis. A second objection of Schultz-Sellack is raised against the possibility of the satisfactory production of colors by a mechanical division of the layer brought about by the exposure, the degree of which would be determined not by the colors but by the intensity of the light. The coloring would, under these circumstances, be only accidental. This explanation is in reality shown to be erroneous in chapter five. New doubt concerning the general validity of Zenker's theory is raised by the investigations of Cary Lea on the haloid salts of silver. He showed that the colored substances produced by the action of colored light on chloride of silver already exposed may be produced by purely chemical methods in the dark. H. Krone also has recently given a series of reactions for the Poitevin process which are carried through by the exposure, and by which different colored bodies may be produced in the sensitive substances by purely chemical means. He announces, therefore, "The method of Poitevin rests upon purely chemical processes," and is "totally dif ferent from that of Lippman." But he also makes the following remark: "This causal connection "-namely, between the color of the light and the above-mentioned bodies which may be produced by chemical means-"is of a purely physical nature, and is only to be explained with reference to the processes and by the progress of investigation upon wave motion and the nature of light." If, now, the Zenker theory has no application in this case, it is not expressly stated wherein the verification fails. On the contrary, Krone asserts" that our present knowledge of the method of photographic production of colors, so far as this has until now been chiefly deduced, Goethe Farbenlehre 2: p. 716. The there communicated memoir of Seebeck I found in none of the editions of the complete works of Goethe which I could command. Carey Lea: "On red and purple chloride, bromide, and iodide of siver; on heliochromy and on the latent photographic image." American Journal of Science, series 3, 33; p. 349, 1887. In an address published in the Deutsche Photographen-Zeitung, p. 327, ff. 1891, and in his book "Darstellung der natürlichen Farben durch Photographie,” published by the Deutsche Photographen-Zeitung, p. 43, 1894. In the beginning of the same address. Loc. cit., p. 49. "In his book, p. 38. rests upon Zenker's theory." He implies that the knowledge is not as yet firmly established, and this may be inferred also from the following sentence, which finishes the treatise above cited: "We may assume, in consideration of the color processes of which we have been speaking”—those which I have designated as old-"that the resulting colors appear the same to us as the colored lights used in the exposure, because the molecules of the layer exposed continue to vibrate with the same wave lengths which they encountered in the light to which they were subjected." The assumption thus made in the last sentence leads however to no explanation of the color results; for since the place exposed does not become self-luminous, the further vibrations of the molecules must result in the absorption of the colors before illuminating and the place would appear of the colors complementary to these. In this state of affairs the fundamental question must first of all be proposed: Are the colors appearing in the older processes apparent or body colors-that is to say, produced by interference or absorption? In the first case, which is the one required by Zenker's theory, it must be further asked: How is it then possible that the same colors may be produced by chemical means? Is it possible that by chemical action a body may be produced with a stratified structure which is capable of producing interference? Krone indeed alleges this extraordinary possibility; and it becomes clear that in this case one may distinguish a process as chemical without contradicting Zenker's theory. In the second case however Zenker's theory is not applicable, and one is confronted by the remarkable, and for my science new, issue, the fundamental possibility that colored illumination can create corresponding body colors. Yet in these circumstances one might perhaps fall back on the consideration that absorption and interference may not be fundamentally different. Thus absorption follows from the interference theory developed by Wrede. Such an assumption is not however compatible with the fact that the metals show the characteristics of absorption at a thickness of about the wave length of light. It is fundamentally contradicted moreover, as was shown long ago by Stokes and Rudberg, by the fact that absorption is connected with a loss of light whose energy is changed into other forms, as for example, into heat or chemical energy, while with interference alone no light is lost, the reflected and transmitted together being always equal to that incident, and in the case of white light complementary to each other. But why is it that the question as to the source of the colors in the older methods of color photography has not already been easily decided? Zenker gives the answer to this question. The fundamental substance in these is chloride of silver or a lower chlorine compound of this salt. In the address above cited. 100 2 Wrede's theory and refutation, see Wüllner, Lehrbuch der Experimental-Physik 2: p. 456, 4th edition, 1883. 3Zenker. Photochromie, p. 85. The index of refraction for pure chloride of silver is about 2, and for compounds poorer in chlorine probably even greater. When therefore a ray of light falls upon silver chloride even with a considerable angle of incidence, it would pass within nearly perpendicular to the surface, on account of the great index of refraction, and the difference in path for interference as compared with direct incidence is only slightly changed. The index of refraction of the sensitive layer in Lippmann's process on the other hand, which consists chiefly of collodion or gelatine, is only about 1.5. Here the angle of incidence becomes of importance, and the colors change with it in a way not to be desired. The interference nature of the colors and the stratification of the Lippman gelatine plates may be recognized in another way. Different observers breathed on such plates and saw appear in place of the original colors others of greater wave-lengths. This showed that the colors depend on a changeable distance, namely, that between the elementary mirrors within, which was increased by the swelling of the gelatin.2 I have repeated this experiment, and before large audiences have replaced the breathing on the plates by the use of a stream of vapor. In the photograph of the spectrum thrown upon the wall the colors were transformed with great rapidity in the direction of the violet end of the spectrum and returned again as the moisture was driven from the gelatin film by a Bunsen burner. This experiment can not, however, be performed with the older processes. Zenker sought to give the light rays in chloride of silver a greater angle with the normal to the surface by sending them first through a liquid of high index of refraction, but without result. This expedient must fail so long as a plane parallel layer of such a substance is used. For by the refraction in this the original angle of incidence is decreased. Such a diminution can not occur when the beam of light enters the bounding surface of the auxiliary substance at right angles. Thus a new experiment was suggested. I used a right angled glass prism with an index of refraction of 1.75 for the D lines. This was laid with its hypotenuse surface upon the color picture and the intervening air space filled up with a layer of benzine. For light rays entering normal to the side surfaces an angle of incidence of 45° is thus secured in the strongly refracting medium, and the ray entering the chloride of silver must, therefore, form a consider 'Meslin, Ann. de chim, et de phys. (6), 27, p. 381, 1892; Krone, “Darstelling der natürlichen Farben," p. 66; Valenta, "Die Photographie in natürlichen Farben," p. 68; Halle a. S., published by Wilh. Knapp, 1894. Dr. Neuhauss (Photogr. Rundschau, p. 295, 1895) mistakenly believed that he had found an objection to the applicability of Zenker's theory to Lippmann's process in the observed magnitude of the grains in the undeveloped plates, which was found to be 0.0003 millimeter. Not the absence of grains, as he supposes, but complete transparency is requisite for the production of fixed light waves. 3 Zenker. Photochromie, p. 85. |