THE VISUAL PIGMENTS 



significance of a pigment cannot be assumed until corresponding 

 retinal or behavioural activity has been demonstrated. 



Recently, new methods of investigation have been developed. 

 These, whilst having difficulties of their own, are free from some of 

 the above-mentioned disadvantages. 



SUSPENSIONS OF VISUAL CELLS 



As described in Chapter 1, the outer-segments of the retinal rods 

 (and, presumably, of cones also) can be suspended in a 35-40 per 

 cent sucrose solution, arden (1954a) had the idea of using such 

 suspensions as working material. If practicable, this offered a means 

 of studying the visual pigments under conditions which more nearly 

 approached their natural environment. Since most of our knowledge 

 relates to the visual purple of frogs, arden decided to work with 

 these animals. This was a fortunate choice for suspensions of frog 

 retinal outer-segments are stable, and retain their visual pigment for 

 at least a week. This behaviour may be contrasted with that of some 

 fish outer-segment suspensions which last only a few hours (dart- 

 NALL, quoted by arden, 1954a). 



photometric investigation of suspensions 



At first sight these suspensions would not seem to be very suitable 

 material for exact spectrophotometric work. They scatter light very 

 strongly and, in consequence, have a milky appearance, in which, 

 however, can be detected a pinkish colour (changing to yellow after 

 exposure to fight) due to the visual pigment. Largely because of this 

 scattering, such preparations transmit only a small proportion of the 

 incident light (e.g. when of 0-5 cm thickness they transmit 1 in 250 of 

 violet light and 1 in 25 of red fight). 



Optical density is defined as, 



intensity of incident light 

 intensity of transmitted light 



and consequently light which is scattered has the same effect, so far 

 as the measurements are concerned, as that which is truly absorbed. 

 The measured density is thus due in part to absorption but mainly to 

 scattering. A typical suspension would have a density at 500 m// of 

 2-0 before exposure to fight and 1-8 after. The transmissions corre- 

 sponding to these figures are 1 per cent before bleaching and 1-7 per 

 cent after. Percentage transmissions of this magnitude cannot be 



182 



logio 



