212 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1953 



could be photographed to somewhat beyond 13,000 A. Each of these 

 steps of progress made it much easier, of course, to photograph the 

 shorter wavelength portions of the infrared spectrum, so that at the 

 present time photography in the infrared between 8000 A. and 9000 A. 

 presents little more difficulty than photography in the visible spec- 

 trum. Only when it is necessary to stretch sensitivity as far as possi- 

 ble into the infrared is any difficulty encountered with the sensitizing 

 dyes. 



Quite recently an important improvement has been made in the 

 preparation of dyes sensitizing in the region between 9000 and 

 12,000 A. For photography of the spectrum beyond 9000 A., the dyes 

 available are the tetracarbocyanines and pentacarbocyanines. A typi- 

 cal pentacarbocyanine is shown at the bottom of figure 1, in which the 

 two nuclei are connected by a conjugate chain containing no less than 

 11 methine groups. Unfortunately, this very long chain is easily 

 broken, so that the dye is extremely unstable, and until recently the 

 spectrum beyond 9000 A. could be photographed only with intense 

 sources, such as that of the sun. It was almost impossible to photo- 

 graph stellar spectra in this region, and little success had been 

 achieved even in the photography of the solar surface by the spec- 

 troheliograph using the important helium line at AlO,830. 



The organic chemists in our laboratory have now found a way of 

 modifying the structure of a pentacarbocyanine dye to obtain greatly 

 enhanced stability so that the dye can be purified and used in an 

 essentially pure condition. Using plates made with this new dye, 

 F. D. Miller has been able to obtain a number of spectra of late- 

 type stars using an objective prism on a 24- to 36-inch Schmidt tele- 

 scope. The infrared limit is somewhat beyond 11,000 A. A strong 

 absorption band at 9300 A. is an atmospheric band due to water vapor, 

 but a considerable group of absorption bands have been found in the 

 spectra of N-type stars. Up to the present the molecules responsible 

 for these bands have not been identified. It is believed that the new 

 Z-type plates will make it possible to obtain spectroheliograms using 

 the helium line at 10,830 A. 



While the photographic spectrum has been extended by the dis- 

 covery of new infrared sensitizers, there has also been a great increase 

 in the sensitivity of photographic materials to the red region of the 

 spectrum, which has made it possible to make stellar photographs by 

 red light with exposures not greatly in excess of those required with 

 the ordinary violet-sensitive materials. It will be recalled that the 

 different classes of stars are of different colors, and though a number 

 of stars are definitely bluer, a very large number of stars are distinctly 

 redder than the sun. In the older photogi'aphs, taken on materials 

 sensitive to only the blue and ultraviolet rays, these red stars were 



