[lubovich & pearen] INFRA-RED SPECl'ROSCOPY 201 



experimentally known properties of the two d^'es, (1) that Dicyanin A 

 is better than Dicyanin for longer wave-lengths and (2) that it must 

 be taken for a staining bath in a little greater concentration. 



According to its absorption curve Dicyanin A is better for work 

 at 0.9 At than Dicyanin, but the latter is preferable for 1 jj. although 

 Dicyanin A again has an advantage in account of its greater ab- 

 sorption. 



Fig. 4 shows the absorption for Pinacyanol. This dye has a 

 much greater percentage absorption at . 5 /z than the dyes previously 

 mentioned. At the same time our results indicate that it would 

 work as well as both Dicyanins up to 0.9 /z. The first of these facts 

 is well known, but the second needs experimental verification. 



Fig. 5 represents the absorption curve for Nigrosin SS Blue 

 shade. This dye, according to its absorption, should give better 

 results for work between 1 .0 At and 1 . 1 m than Dicyanin. 



Fig. 6 shows the absorption for Alizarin Blue S. This dye has 

 no maximum at 0.6 /i-O.T n, in which it differs from all the dyes 

 mentioned above. Possibly it can be useful at 1.0 m. thanks to its 

 well marked maximum in this region. 



Fig. 7 shows the percentage absorption of a dye recently brought 

 out by Dr. Mees of the Eastman Kodak Co., and known as red 

 sensitiser No. 700. As the diagram indicates there was a relatively 

 strong but narrow band at 0.6 m- In the region between 1.2 /x and 

 1 .9 Ai the relative absorption was exceptionally high. If one can take 

 absorption as an indication of photographic sensitivity this dye 

 should prove very useful in infra-red work up to 2 }x. 



Fig. 8 represents the absorption for a 5 mm. thick layer of ethyl 

 alcohol. 



The study of the filters is represented in Figs. 9-14, which show 

 the relative per .cent, transmission of the following filters: Wratten 

 No. 22, No. 29 + No. 22, No. 70 + No. 22, No. 36 + No. 22, No. 29 + 

 No. 45 (Paschen's filter), and 0.1075 mm. thick layer of asphaltum 

 varnish, deposited on a glass 1 . 1 mm. thick. 



Each of the first four filters was studied twice. Curves A 

 represent the transmission of the filter in question and Curve B the 

 transmission of the given filter plus a plate of glass 3.5 mm. thick, 

 which was added to absorb the radiations of very short wave-lengths 

 in case the filter was transparent to them. 



Fig. 9 shows that filter No. 22 has exceptionally high transparency 

 for near infra-red radiation ranging from 86 to 89 per cent. Other 

 filters have 70 per cent, as an average transmission. 



