[lubovich & pearen] INFRA-RED SPECTROSCOPY 197 



region to be investigated, a differential method was adopted, involving 

 the use of a thermopile and galvanometer. 



The deflection di was taken when the radiation was passed 

 through a given thickness of alcohol and an instant later the deflection 

 É?2 was taken when the radiation was passed through an equal thickness 

 of dye solution. In this way the difference di-di was obtained. The 

 deflection d, when the radiation was passed directly into the spectro- 

 meter without any absorption cell was then observed. The readings 

 for di, di and d were taken at each 0. V of the spectral region investi- 

 gated at least five times, or until consistent results were obtained. 

 The percentage relative.absorption was calculated from the equation 



Absorption = — v~ XlOO. 



As to the investigation of filters it has been pointed out that the study 

 was proposed from the point of view of the need of infra-red spectro- 

 scopy, which involves two questions: 



1. How great is the transmission of a filter in the red and infra- 

 red? 



2. Is there a narrow region of transmission for short wave- 

 lengths which might be a source of error when the filter is used with 

 a grating? 



As far as transmission is concerned, a method of study was used 

 similar to that adopted in the study of the relative absorption of the 

 dyes, with the difference that the deflection di was taken when the 

 radiation passed through a given filter, di when the radiation passed 

 through a given filter plus a plate of glass 3 .5 mm. thick, and d when 

 the radiation passed into the instrument without any filter in its path. 

 The per cent, relative transmission was calculated from the equations. 



Transmissions of a filter = ~T X 100 



d 



di 

 Transmission of a filter plus glass ~7 X 100. 



Different Wratten light filters were studied, and the solution 

 of problem (2) obtained in the following way: 



The spectrum of mercury was photographed four times on one 

 plate — 



(a) Without any filter. 



(b) Through the first filter of the proposed combination. 



(c) Through the second filter, which was intended to cut ofif 

 some undesirable lines. 



(d) Through the combination. 



