142 COLOUR VISION 



towards the violet end, much as it would be seen by the human eye 

 through a reddish yellow glass. The shortening is similar but rather 

 less for nocturnal birds. Both types possess a power of dark adaptation 

 the same as or very similar to that of man. 



Reptiles. Hess investigated several varieties of tortoise, and found 

 that their vision nearly resembles that of birds. They have the same 

 shortening of the spectrum towards the violet end. More remarkable 

 is the fact that they possess a high degree of dark adaptation, the 

 increase in sensibility being seemingly the same as that of the human eye 

 looking through a suitable orange-tinged glass. It may be mentioned 

 here that these animals are generally admitted to possess no rods or 

 visual purple in their retinae. 



Hess noticed that the only animals which he examined and found 

 to possess a shortened spectrum towards the violet end were such as 

 possess coloured oil globules in their cones. At the same time he points 

 out the difficulties in accepting this fact as a complete and satisfactory 

 explanation. 



Amphibia. Yerkes^ caused a green frog to go through a simple 

 labyrinth leading to a tank of water. At the point where the first 

 choice between two paths occurred a red card was placed on one side 

 and a white on the other. When the frog had learned to take the 

 correct path the cards were exchanged and the confusion of the animals 

 showed that they discriminated between the cards, though it may have 

 been a luminosity- rather than a colour-discrimination. Loeb^ found 

 that frogs showed negative phototropism, and preferred red to blue light. 

 Torelle^ found that two species of frog showed positive phototropism, 

 associated with a tendency to prefer blue to red light. The phototrop- 

 ism persists when the animal is blinded, although in the normal animal 

 the eyes are involved in the reaction since it occurs when the skin is 

 covered and the eyes left intact*. Cole^ found that the phototropism 

 of Rana clamata differs according to the surrounding temperature. 

 Pearse^ found some amphibia positively, and others negatively helio- 

 tropic. 



1 Harvard I'.sijch. Studies, i. 570, 11)03. 

 - Der HcUotropismuH der Tie.re, Wiirzburg, 1890. 

 3 Amcr. J. oj Phymol. ix. 466, 1903. 



* Koranyi, Centralbl.f. Physiol, vi. 6, 1892 ; G. H. Parker, Amcr. J. of Physiol, x. 28, 

 1903. 



5 J. ofComp. Neur. and Psychol, xvii. 193, 1907. 



" Proc. of the Amcr. Acad, of Arts and Sc. xlv. 16 J, 1910. 



