THE VISUAL PIGMENTS 



focussed on to the animars fundus, F. After their respective reflec- 

 tions from these surfaces, the two beams were brought alongside each 

 other once more by the action of the cube P, and focussed by L^ in 

 the plane of the observer's pupil. The observer saw a T circular 

 field, one half of which was due to the matching beam and the other 

 half due to the test beam. By moving the neutral wedge, W^ the 

 intensity of the matching beam could be equated to that of the test 

 beam. 



RESULTS WITH ANIMALS 



Applications of rushton's apparatus (Fig. 7.5) to the measure- 

 ment of changes in the retinae of the cat and rabbit have been 

 described in a number of brief communications (rushton, 195?, 

 1953a, 1953b; hagins and rushton, 1953). The present account is 

 drawn from a recent summarizing paper (rushton, Campbell, 

 HAGINS and BRINDLEY, 1955). 



These experiments were carried out on a decerebrate and dark- 

 adapted rabbit. First, at each of ten different wavelengths, positions 

 of the wedge, W (Fig. 7.5) were determined at which agreement 

 between the test and comparison beams was obtained. A bright 

 green light was then shone into the eye and the wedge densities in 

 light of 5 1 7 m/i recorded every 1 5 or 30 sec. The results obtained 

 (empty circles, Fig. 7.7 (A)) give the time course of bleaching. When 

 bleaching was complete, or nearly so, the wedge positions for equahty 

 of test and comparison beams at each of the ten wavelengths were 

 again determined. The differences between the two sets of readings, 

 i.e. for the dark- and light-adapted retina respectively, are shown in 

 Fig. 7.7 (B) where they are compared with data for visual purple. 

 Finally the course of regeneration was measured by switching off the 

 bleaching light and taking wedge readings at regular intervals (filled 

 circles, Fig. 7.7 (A)). These measurements were made in hght of 

 517 rafi apphed for about 5 sec every 5 min. Tests showed that in 

 5 sec the measuring fight caused less than 0-0002 density unit of 

 bleaching. 



Results of a similar nature were also obtained by we ale. Thus 

 WE ALE (1953b) recorded the regeneration of visual pigment in the 

 retinas of fight-adapted, decerebrate cats. In most cases the rate of 

 regeneration was slow, the half-return period being about 20 min 

 (compare Fig. 7.7 (A)). The difference spectrum for the slow changes 

 had a maximum at about 495 m/z (Fig. 7.8) and indicated that, in the 



198 



