174 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 30 



Abney obtained his results with a special form of silver bromide 

 which had the power of absorbing red and infra-red radiation; he 

 used no dyes. Pie prepared his own photographic plates shortly be- 

 fore use by an elaborate process which he described in detail. But 

 his process is troublesome and uncertain. It has never been seriously 

 applied to modern spectroscopy, although the resources of the present 

 chemical laboratory would perhaps warrant a revival of it. Since 

 Abney's day, photographic chemistry has been more concerned with 

 ordinary silver bromide, which of itself absorbs only blue, violet, 

 and ultra-violet light. To make this bromide useful for longer wave 

 lengths, dyes are added to the emulsion which, by their absorption 

 of the yellow, red, or infra-red, are designed to confer sensitivity 

 to those kinds of radiation. This is the method used in preparing 

 panchromatic emulsions with which many of you are no doubt famil- 

 iar. Commercial needs have so stimulated the perfection of such 

 emulsions that those now obtainable are of excellent quality. For 

 the infra-red, however, the commercial requirements have not been 

 so great and progress has been much slower. For many years only 

 one useful dye was known for this region. It is called dicyanine 

 and it was with this that Doctor Brackett obtained his excellent 

 results some years ago. Lately, however, the motion-picture indus- 

 try developed a need for emulsions which would permit the taking 

 of night scenes in daj'time, since great expense could thus be saved. 

 Now it turns out that a picture made with infra-red light under 

 ordinary daylight conditions looks very much as though it has been 

 made at night. In such a picture the sky, for example, is dark in- 

 stead of light. (See pi. 1.) Perhaps then, the spectroscopist owes 

 something to the movies for the fact that not long ago two new dyes 

 were produced by the Eastman Kodak Laboratory under Doctor 

 Mees's direction. One of these is particularly important for our 

 purpose, and it is the outcome of using it that we shall now hear. 

 This new dye is called neocyanine. It is produced in the form of 

 beautiful red crystals which are soluble in alcohol. An ordinary 

 photographic plate soaked in this solution acquires infra-red sensi- 

 tivity far beyond that obtainable with other dyes. It has thus be- 

 come feasible to use instrumental power which is comparable with 

 that employed in the visible part of the spectrum and sufficient to 

 bring out a wealth of detail beyond the power of the thermometric 

 methods. 



For this purpose the solar spectrum is formed by an optical 

 instrument called a concave grating, especially constructed to give 

 exceptional brightness to the spectrum. A small image of the sun 

 is formed on the slit of the spectrograph which is covered by a 

 filter. This stops the visible light and transmits only the invisible 



