INTRODUCTION AND METHODS 



been the measurement of spectral sensitivities. Such measurements 

 consist of finding out how the amount of hght energy, needed to 

 evoke a given constant response, depends on the wavelength of the 

 light. 



In human subjects equahty of sensation can be used as the criterion 

 for equality of response. For example, the subject can be asked to 

 say when two Ughts of different wavelengths seem equally bright to 

 him. 



With animals similar information can be obtained (though with 

 less precision) by observing their behaviour under controlled condi- 

 tions. Thus the spectral sensitivity of the South African clawed toad 

 {Xenopus laevis) has been measured (denton and pirenne, 1951) 

 by noting the light intensities below which the animals failed to go 

 into the dark part of their tank, it being a characteristic of Xenopus 

 to seek the shade. 



When there is no marked natural phototropism, the animal can 

 sometimes be trained or conditioned — depending on its intelligence — 

 to respond to a hght stimulus. Thus gunter (1952, 1953) trained 

 cats to raise and pass through a hinged panel which was illuminated 

 by white hght of feeble intensity in preference to one which was 

 brightly Ht. After their training, the animals were required to make 

 a choice between a panel which was ht by feeble white hght and one 

 which was ht by coloured Hght, the wavelength and intensity of which 

 could be varied. By finding at what intensities the coloured stimuli 

 were indistinguishable (to the cats) from the constant white stimulus, 

 GUNTER was able to obtain a spectral sensitivity curve. 



In the above examples the conscious or subconscious equation of 

 sensations is the index for equality of response. But other criteria can 

 sometimes be employed ; for example the electric changes occurring 

 in the nervous layers of the retina. 



Pre-eminent among workers in this field is the Swedish physiologist 

 RAGNAR GRANiT who used a microclectrode method for studying 

 the changes which occurred in the retina when it was stimulated by 

 hght. 



In granit's spectral sensitivity measurements the pre-retinal 

 media were first removed from the eyes in order to allow a micro- 

 electrode to be placed in contact with a point on the inner surface of 

 the retina, rushton (1949, 1950) has shown that the electrical 

 effects picked up by the microelectrode had their origin in the large 

 ganghon ceUs of the retina (Fig. 1.2). 



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