ASTRONOMICAL PHOTOGRAPHY — MEES 209 



Though the improvement in sensitivity of photographic materials 

 has depended upon a study of the general principles of emulsion mak- 

 ing, great progress has been made in another direction. Before 1900, 

 the photography by which the spectra of the stars was studied was 

 confined almost entirely to a limited region of the spectrum. Ordi- 

 nary photographic plates are sensitive to only the blue and violet and 

 ultraviolet regions of the spectrum. Some 10 miles above the earth 

 there is a layer that contains ozone, formed by the action of short-wave 

 ultraviolet light upon the oxygen, in sufficient concentration to ab- 

 sorb practically all the ultraviolet of shorter wavelength than 3000 A. 

 At the same time, the ordinary silver-bromide plate cannot record 

 light of longer wavelength than 5000 A., and thus the astronomer's 

 range of the spectrum was originally confined to the spectral regions 

 between 3000 A. and 5000 A. In 1873 Heinrich Vogel discovered that 

 silver bromide could be dyed and that some dyes made the bromide 

 sensitive to the spectral regions corresponding to their absorption 

 bands. 



By the use of cyanine dyes made in Germany in the early years of 

 this century, it was possible to sensitize photographic emulsions to 

 the whole of the visible spectrum, and in 1906 commercial panchro- 

 matic plates were manufactured. They were very useful for the 

 photography of spectra, and astronomers often sensitized their own 

 plates by bathing them in solutions of dyes. Curiously, the German 

 chemists who discovered these dyes did not determine their chemical 

 structure, and it was not until 1922 that the general structure of the 

 cyanine dyes was understood, largely as a result of work done in the 

 chemical laboratories of Cambridge University. Many new dyes 

 could be prepared, some of which were found to be excellent sensitizers. 

 Photographic manufacturers in the United States, England, and Ger- 

 many then started to prepare new sensitizing dyes belonging to, or 

 related to, the cyanine series. 



At this point there arose a problem which is frequently observed 

 in the application of scientific research. It was comparatively easy 

 to synthesize dyes of the cyanine type that would probably be sensi- 

 tizers, but it was necessary to test the dyes adequately for their sensi- 

 tizing power by adding them to emulsions of which coatings would 

 have to be made. Different emulsions take dyes differently, so that 

 it was desirable to test each dye in several emulsions. Moreover, it was 

 soon observed that the behavior of the dyes when used together in 

 the same emulsion was by no means additive. Some dyes greatly 

 enhanced the sensitizing effect of others; some disagreed with their 

 fellows. A well-organized study would require the trial of each dye 

 with every other dye in pairs at least and for several emulsions; and, 

 in some cases, the use of three or four dyes together. This presented 



