OTHER METHODS OF STUDYING VISUAL PIGMENTS 



measurements made with light transmitted by an Ilford 604 filter 

 (maximum transmission at 520 m//). 



From the known absorption spectrum of frogs' visual purple and 

 the spectral transmission of the filter, denton and wyllie calculated 

 that to obtain the corresponding density at the maximum (500 mju) 

 these values should be increased by 15-7 per cent, i.e. to 0-76 for the 

 individual rods and to 0-52 for the retinal average. This calculation 

 assumes that the photoproduct in denton and wyllie' s experi- 

 ments does not appreciably absorb light of wavelength 520 mju. If, 

 however, the product is the same as that in neutral solution a further 

 increase of 9 per cent in these figures would be required. 



These values are a good deal higher than have been previously 

 quoted for frogs. Thus wald (1938), after extracting the visual 

 purple from bull-frog retinae {Rana catesbiana) and measuring its 

 absorption in solution, calculated that it would have a density at 

 500 m^M of 0-21 in situ, if spread uniformly over the retina, dartnall 

 (1953) arrived at a similar figure (0-25) for the visual purple of R. 

 temporaria. hubbard (1954), calculated from broda, goodeve and 

 lythgoe's (1938) data that, in R. esculenta, the density at 500 m/^ 

 along the axis of the outer limb of a rod was 0-50. 



The difference between wald and dartnall' s values on the one 

 hand and hubbard's on the other is due to the fact that the rods do 

 not cover the whole area of the retina, denton and wyllie were 

 able to make a quantitative allowance for this. Thus the rods cover 

 only 59 per cent of the retinal surface, and consequently wald's and 

 dartnall's values of 0-21 and 0-25 should be multiplied by 1/0-59 

 giving 0-36 and 0-42 respectively, in comparison with hubbard's 

 value of 0-50. 



Yet a further factor needs to be considered before these results can 

 be compared with denton and wyllie's. The molecules of visual 

 purple are regularly oriented in the rods, denton and wyllie 

 estimated that the oriented visual purple molecules would absorb 

 50 per cent more fight than they would in a random state (e.g. in 

 solution). This factor, applied to the results of wald, dartnall 

 and HUBBARD give the values 0-53, 0-63 and 0-75, respectively, in 

 comparison with denton and wyllie's value, 0-76. 



THE GREEN RODS 



In the photographs of unbleached and bleached retinae (see, for 

 example, Plate 7.1) the spaces between rods and the visual purple 



189 



