TtiE VISUAL PIGMENTS 



through a colHmating lens, Z^, and one of a set of interference filters, 

 F. It then fell upon a glass plate, Gj, at the polarizing angle. The 

 reflected (and hence completely plane polarized) portion of the beam 

 was focussed by Lg upon the cornea of the rabbit's eye and illumined 

 a circular patch of retina. The unwanted light, specularly reflected 

 from the corneal surface and from L^, was plane polarized and was 

 extinguished by a suitably orientated polaroid, P^- The wanted Hght, 

 however, since it had been scattered back from behind the retina, was 

 depolarized. Consequently half of it was in the sense to pass both 

 Gi and P^ and hence to be focussed by L3 on to the cathode of a 

 photoelectric cell, T. 



The comparison beam (that portion of the light from S which was 

 transmitted by the plate, G^ was reflected by the mirror, Af, and 

 focussed by L4 on to the glass plate G2 and so to the cathode of T, 



A rotating polaroid, P^, was situated in front of the source, S. At 

 one angular position of P^, no light was reflected from G^ into the eye. 

 At an angle d from this position the light proceeding into the eye was 

 proportional to sin^ 0, while that proceeding along the comparison 

 pathway was proportional to cos^ Q. The total light received by the 

 photocell was therefore 



I=a sin2 d -\-b cos2 d 



where, for a given wavelength, a was a factor which included the 

 transmissivity of the retina and b, one which included the transmis- 

 sivity of the neutral wedge, W. By moving the wedge, b could be 

 adjusted to be equal to a and then, 



/ = ^(sin^ d + cos^ Q) = a 



The photocell output was condenser-coupled to an oscilloscope. 

 The polaroid, P^, rotated at about 40 rev/sec so that an 80 cycle A.C. 

 wave was normally seen on the screen. By moving the wedge, W, the 

 ampHtude of this wave could be reduced to zero ; at which point, b 

 was equal to a. The wedge reading was then recorded. Any change 

 in the optical density of the visual pigment in the retina caused a 

 change in a with the consequent reappearance of a sine wave upon 

 the screen. By making an equal change in b, however, the balance 

 could be restored. Since the test beam passed twice through the 

 retina, and the comparison beam twice through the wedge, any 

 change in visual pigment density was measured directly by the change 

 in wedge density required to eUminate the sine wave. 



196 



