THE ANATOMICAL BASIS 



15 



in fowls and pigeons, but Boll, Angelucci, and van Genderen Stort 

 found it in pigeons. Hess dissolved out the oil globules with benzol, 

 which does not affect the purple in frog's retina, and found traces of 

 rhodopsin in both fowls and pigeons, but in far less amount than in man, 

 ox, frog, etc. There was more present in the hawk, buzzard, goose and 

 duck. Amongst reptiles the crocodile possesses many rods and is rich 

 in visual purple. 



Electrical Changes. The retina, connected through non-polarisable 

 electrodes with a galvanometer or capillary electrometer, shows a 

 " current of rest " (du Bois-Reymond). On exposure to light there is 

 usually a short negative variation followed by a longer positive variation, 



± r- 



I" *■ 3' •« y 0' r ? r Kf 



Fig. 2. Electrical changes in the frog's eye caused by light. Plotted curves from the 

 analyses of three capillary electrometer records. The illumination in the case of the 

 upper record was white light, in that of the middle red light, and in that of the lowest 

 violet light. Abscissae, time after the commencement of the illumination in seconds ; 

 ordinates, the electromotive force in ten thousandths of a volt. (Gotch.) 



VoU 

 •0007 



01 02 &3 &4 &5 06 07 08 09 10 II l« 



Fig. 3. Curves constructed from typical electrometer records of eyeball responses to the 

 light from the red, green, and violet regions of the interference spectrum of the arc light. 

 (Gotch.) 



