OTHER METHODS OF STUDYING VISUAL PIGMENTS 



In WE ale's apparatus (Fig. 7.6) the light emerging from the exit 

 slit, 5, of a monochromator was rendered parallel by the lens L^, and 

 then fell normally on to the face of a glass cube P composed of two 



Fig. 7.6. Apparatus for the measurement of spectral reflectivity of the 

 tapetum and hence of changes in the concentration of visual pigments 

 in the retina. S, exit slit of monochromator; Lj, Lg, L^ and L4, lenses; 

 P, glass cube with half diagonal plane silvered; W, neutral wedge; M, 

 magnesium oxide surface; F, the animal's eye; O, observer's eye. 

 (After Weale, 1953a) 



45° prisms cemented together. Half the surface of the diagonal plane 

 of P had been silvered. Consequently the hght from the monochro- 

 mator was divided into two, a matching beam which was focussed on 

 to the magnesium oxide surface, M, and a test beam which was 



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