116 J. C. DeVOSS and rose ganson 



color-vision to the dogs and the squirrel which they tested. 

 They concluded that 'The tests seem to show a surprising 

 fineness of color-discrimination among the animals tested". 



The chief results of recent experiments on mammalian color- 

 vision have been the refinements in methods achieved through 

 comparison and keen criticism. We cannot here deal with the 

 technique of experiments other than our own, but the following 

 generalizations will indicate the direction which the improvement 

 in methods has taken. 



1 . So great a difference exists between the results obtained 

 from the light-adapted and the dark-adapted eye that those 

 secured under the one condition give us almost no clue to those 

 which may result from the other. 



2 . All secondary criteria must be rigidly excluded. Differ- 

 ences of form, depth, size, texture, and especially differences 

 of brightness between the lights or test papers used are especially 

 to be guarded against. 2 



3. Much stress has been placed on the need of "natural 

 conditions" in testing an animal's vision. It is reasonable to 

 suppose that artificial conditions, like testing the animals in a 

 dark room, or by artificial light, might lead to false views of 

 his visual powers. 



4. The stimulus color may be (a) reflected light from colored 

 objects, usually papers, or (b) transmitted light which may be 

 filtered through colored glasses or colored fluids, or isolated 

 from certain spectra. The first method is used to test the light- 

 adapted eye, or daylight vision, while the second is used chiefly 

 to test the dark-adapted eye, or twilight vision. 



The second class of methods strives to secure different hues 

 of pure or homogeneous light. Both plans seek to find pairs 

 of colors, or colors and grays which will be confused by the 

 animal, assuming that the intensities of the colors for the animal 

 and for human vision may not be the same. 



Many criticisms of the use of colored papers for testing vision 

 have been made. Their surfaces are said to differ greatly owing 

 to accidents of manufacture. It is said to be difficult to bend 

 them around glasses. They do not reflect homogeneous light 

 but overlapping bands, etc. etc. None of these criticisms, 

 however, applies very clearly to pairs of colors which are confused 

 2 See Yerkes, The Dancing Mouse, pp. 91-92, and 151. 



