56 



Biological Stains 



A 



A ^i 



^M 



j£! P Ni 



-^x 



-tv 



Z2^~> Slivered 



Prism P 



1 

 PHOTOlitTE-R- 



PEC'TKOntTLR- 



Fig. 1. Schematic diagram of Hilger-Nutting spectrophotometer. Specimen 

 holder with dye dissolved in a suitable solvent is placed at Ei while a similar cell 

 with solvent alone is placed at Ej. Both are equally illuminated from the same 

 light source. Through the partially silvered prism, P, the upper and lower portion of 

 the photometer field is illuminated through the specimen while the central portion is 

 illuminated through solvent alone. This latter illumination may be varied by the 

 rotation of the analyzer, Nj, which is actuated by a graduated circle C, reading with 

 the index I. By this means the illumination of the several portions of the field is 

 maintained equal and absorption values read off of circle C. The spectrometer of 

 the constant deviation type serves to isolate particular portions of the spectrum by 

 means of a graduated drum D which through a screw actuates the prism table sup- 

 porting prism B. 



This visual type of instrument, as explained above, is now rarely 

 used, having become obsolete on account of the greater efficiency 

 of the photoelectric cell in comparison with the eye. The eye is 

 badly subject to fatigue after continuous use, and even at its best 

 has low sensitivity at the violet end of the spectrum. The photo- 

 electric cell, on the other hand, is not so subject to fatigue, can 

 make readings over a longer wavelength range, and even into the 

 ultra-violet, if special equipment for that purpose is provided. 

 For these reasons more rapid readings can be made, and it is pos- 

 sible to obtain the exact point of maximum absorption much more 

 quickly than with the eye. A photoelectric instrument such as 

 the General Electric Recording Spectrophotometer, is even more 

 automatic, as it has a motordriven device that records even the 

 most complicated absorption curve on a revolving drum in only a 

 few minutes. A recording spectrophotometer, however, is a rather 

 unnecessary refinement in the case of dyes, because of their rela- 

 tively simple absorption curves. Simple non-recording photo- 

 electric instruments, such as the Beckman or the Coleman, are 

 ordinarily suflBcient. 



The optical principles of the Beckman are given in Fig. 2 and 

 a photograph of an assembled instrument in Fig. 3. Details of 



