148 



BIOPHYSICALLY ACTIVE LIGHT 



Fig. IV-10. Diagram of a " Spekker " 

 photometer placed in front of the spectrograph 

 slit G. 



A prism monochromator or prism spectrograph is preferably used to 

 resolve the visible beam of light into its spectrum. For exploring the 

 ultraviolet, the above instruments are provided with quartz lenses and 

 prisms. 



A means of evaluating the relative intensities of the various wave- 

 lengths before and after they have passed through the absorption cell 

 constitutes the photometric phase of the absorption measurements. A 



photoelectric cell or a linear 

 thermocouple may be used for 

 this purpose in connection with 

 a monochromator. The in- 

 tensities are evaluated from a 

 developed photographic plate 

 when a spectrograph is used. 



One of the simpler precision 

 absorption instruments now 

 much in use is the split beam 

 " Spekker " absorption pho- 

 tometer (Twyman and Allsopp [1934]), illustrated in Fig. IV-10. The 

 light source Q, and the " Spekker " photometer containing the absorp- 

 tion cells C\ and C 2 , are adjusted so that the two emerging beams 

 are focused one above the other on the slit of the spectrograph G. 

 The spectrograph resolves these two sources and focuses them as two 

 contacting spectral images on the photographic plate. 



The light from source Q is internally reflected from the faces of the 

 two rhombs Ri and R 2 . Thus two beams of equal intensity are pro- 

 duced. One beam is diverted upward, and the other downward; both 

 are reflected forward through the lenses L\ and L 2 . The upper beam 

 passes through a precision slit S\ and an absorption cell C\ containing 

 the solvent, and thence through the lens L 3 via the rhomb R% into the 

 upper half of the spectrograph slit G. An identical optical path is 

 traced by the downard deflected beam in passing through the absorption 

 cell C 2 containing the solution. The image of the adjustable slit S 2 

 falls below that of Si on the spectrograph slit. The slits Si and S 2 

 control the intensity of the two beams. The slit S 2 is variable in size 

 by means of a precision micrometer push screw. By proper adjustment 

 of the light energy passing through S 2 the photographed spectrum of 

 this beam may be matched for intensity at any desired wavelength with 

 a similar wavelength in the spectrum photographed just above it by 

 means of the light passing through S\. 



Slits aSi and S 2 are illuminated with equal fluxes of uniform radiation, 

 which may be represented by 7 (per unit area). Let Ai and A 2 be the 



