ADVANCED PHYSIOLOGICAL CHEMISTRY 



483 



ing in the region between the Fraunhofer lines G & H vary somewhat 

 in position according to the particular substance examined. 



The Spectrophotometer. The spectrophotometer is an instrument 

 by means of which the amount of light absorbed in any part of the 

 spectrum as the result of passing through some absorbing solution may 

 be measured. The amount of light thus absorbed depends upon the 

 concentration of the solution and the thickness of the absorbing 

 stratum. 



The instrument may be used then for two purposes. ( 1 ) To measure 

 the amount of light absorbed by some medium in any given portion of 

 the spectrum, (2) to compare the concentration of solutions of the same 



276. The Spectrophotometer. 



substance at different strengths, and therefore by comparing the concen- 

 tration of an unknown solution with that of a standard to estimate the 

 strength of such solution. 



Glazebrook's form of the instrument consist of a flat rectangular box. 

 At one end A is a slit. Light passing through this is focussed by the 

 lens L as a parallel pencil on the direct vision prisms SS. At G is a 

 second slit, and light passing through this is directed by the mirror K 

 to form a parallel pencil below that from A upon the prisms. By 

 adjusting K the spectra may be made to coincide. The emergent 

 beams are focussed by M and the line of separation may be made 

 distinct by movement of the eyepiece H. At G and F are polarising 

 Nicol's prisms, F being its principal axis vertical, G horizontal. An 

 analysing Nicol is at H, and when the pointer of the rotating eyepiece is 

 at zero the principal axis of this is vertical. The light passing through 

 A is wholly transmitted through the analyser at positions and 1 80, 

 that from G is totally extinguished. At 90 and 27(X the whole of that 

 from G passes, but none from A. By means of a sliding slit at B any 

 desired portion of the spectrum may be viewed. Between and 90 



