368 STUDIES IN SPECIAL SENSE PHYSIOLOGY 



dark adaptation, the tertiary gains in brightness and loses in 

 chromatic value ; indeed, owing to the high adaptive powers of blue 

 and green lights, when these are used at moderate intensities, the 

 colouration of the tertiary image is only visible at the beginning 

 of the experiment. 



There is some difference of opinion as to whether the tertiary 

 can be obtained in direct vision ; since red light is the most suit- 

 able stimulus for calling up this image, it is reasonable to suppose 

 that it should be perceived by direct fixation. The suggestion is, 

 therefore, that the customary form of the tertiary image is due to 

 a chromatic element, unaffected by dark adaptation, and a bright- 

 ness element which is so affected. 



We are almost in a position to attempt a sorting out of the 

 various experimental facts described ; only one group remains 

 for examination, namely, some researches into peripheral changes 

 which accompany the retinal alterations, and are important as 

 objective signs of the latter. 



Schirmer asserted that the pupil width and its reaction were 

 related to the adaptive condition of the eye. With complete 

 adaptation to a given grade of light, the pupil reaches after initial 

 widening or narrowing a physiological mean position. Garten ( 23 ) 

 found that momentary illumination produces in the " light " eye 

 a weak sudden, in the " dark " eye a slow powerful contraction. 

 Proceeding further on these lines, Sachs ( 23 ) discovered that the 

 pupillo-motor response to coloured lights followed closely their 

 adaptation values. Abelsdorff ( 23 ) confirmed these results, using 

 the apparatus sketched (Fig. 3). One of the two lights serves as a 

 standard ; the subject looking through the strong convex lens at the 

 slit sees a bright point surrounded by a diffusion circle. If the light 

 falling on the eye be changed the diffusion circle increases or 

 diminishes in size. By rotation of a Nicol prism the new intensity 

 can be increased or diminished and the intensity is found for which 

 the diffusion circle produced by the standard light is not increased 

 or diminished by changing to the tested colour. Naturally, the 

 method does not yield very exact quantitative measurements ; the 

 mean error is said to be about 7 per cent. The next table (see 

 p. 370) gives some of Abelsdorff's results. 



The close agreement between the pupillo-motor values and 

 those of apparent brightness justifies the method, and its importance 

 lies in the fact that we can employ it in experiments on animals. 



