OTHER METHODS OF STUDYING VISUAL PIGMENTS 



measured with the desired accuracy. At longer wavelengths the 

 situation improves a Httle, but at shorter wavelengths it rapidly 

 worsens. Fortunately, this practical difficulty can be easily overcome 

 by measuring the density of a suspension with reference to another 

 sample of the same suspension. If the two optical cells are exactly 

 the same and the samples are truly identical, zero density will be 

 obtained at all wavelengths. If, now, one of the samples is exposed 



500 

 Wave-length (m//) 



Fig. 7.1. The density-changes on bleaching a fresh suspension of frog 

 visual cells. (Measurements made with respect to another portion of 

 the same suspension.) A-B and B-C, two sets of interlaced measure- 

 ments giving the 'base-line,' i.e. the initial difference between the two 

 suspension samples. D-E and E-F, immediately after one sample had 

 been bleached by white light. H-I and /-/, 3 hr later. 

 (Arden, 1954a) 



to light and the measurements repeated, only the changes due to the 

 bleaching of the visual pigment will be obtained. This supposes, of 

 course, that the scattering properties of the suspensions are not 

 affected by the bleaching. Direct measurement of the difference 

 spectra in this way has the additional advantage that any small 

 instability of the suspension is compensated by similar changes in the 

 reference sample. 



An example of these measurements is given in Fig. 7. 1 . Initially 

 the difference between two suspension samples was small and neutral 

 {A-By B-Cy Fig. 7.1). After exposing one of them to white Ught for 



183 



