210 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1953 



a tremendous experimental program, but it was possible to carry it out 

 by the use of a special laboratory and equipment with which coatings 

 could be made at the rate of more than 100 a day. The organic 

 chemists of the Kodak Company alone have made over 7,000 dyes in the 

 last 20 years, and though it has not been possible to test all the dyes in 

 combination with all other dyes, an adequate testing program has been 

 carried out by a process of selection and elimination. 



From a photographic point of view, the spectrum may be divided 

 into four regions : The region from the ultraviolet to 5000 A., which 

 can be photographed on plates containing no dye sensitizers ; the region 

 from 5000 A. to 7000 A., which we may call the visible spectrum and 

 which can be photographed on panchromatic materials with short 

 exposures; the region from 7000 A. to 9000 A., the near infrared, 

 which can be photographed on special materials with exposures 

 greater, but not much greater, than those necessary for the visible 

 spectrum ; and the region beyond 9000 A. 



The sensitizing dyes used for the photography of the spectrum 

 above 5000 A. are cyanine dyes, in which two nuclei formed of rings 

 of atoms and containing basic nitrogen atoms are joined to form a 

 dye by a chain of methine, CH, groups. Heavier nuclei give dyes with 

 absorptions and sensitizing maxima displaced toward longer wave- 

 lengths. Similarly, lengthening of the chain of methine groups joining 

 the nuclei moves the absorptions toward longer wavelengths. The 

 shortest chain consists of one methine group only, and the dyes are 

 known simply as cyanines. The next chain has three methine groups, 

 and the dyes were termed carhocyanines by W. H. Mills and W. J. 

 Pope, who first analyzed the structure of the German dye pinacyanol, 

 discovered by B. Homolka in 1904. Dyes with five methine groups in 

 the chain are known as dicarhocyariines ; those with seven methine 

 groups as tricarbocyanines j with nine methine groups as tetracarbo- 

 cyaninesj and with eleven methine groups as pentacarho cyanines 



(fig.l). 



Figure 2 shows the progress which has been made in the extension 

 of the spectral region for which photography can be employed in 

 practice. At the top is the spectral region, including only the blue, 

 violet, and ultraviolet, which could be photographed on silver-bromide 

 plates without any sensitizer. Then the discovery of color sensitizing 

 by Vogel and particularly the use of erythrosine made it comparatively 

 easy to photograph through the green region of the spectrum and 

 record wavelengths up to approximately 6000 A. 



In 1904 the application of Homolka 's pinacyanol to the production 

 of panchromatic plates made it possible to photograph to the limit 

 of the visible red, a region which may be roughly placed as just beyond 

 7000 A. In 1919 E. Q. Adams and H. L. Haller in Washington dis- 



