The Sense of Sight: Color Vision 155 



sulphate. The green, which, however, was not monochro- 

 matic, was obtained by the use of a filter box which con- 

 tained a saturated solution of nickel nitrate. These three 

 sets of filters were examined spectroscopically both before 

 the experiments had been made and after their completion. 1 

 The red filters, of which I had two for shifting the lights, 

 transmitted only red light. The blue-violet filters, two also, 

 at first appeared to transmit only portions of the blue and 

 violet of the spectrum, but my later examination revealed a 

 trace of green. It is important to note, however, that the 

 red and the blue- violet filters were mutually exclusive in the 

 portions of the spectrum which they transmitted. Of all 

 the filters used the green finally proved the least satisfactory. 

 I detected some yellow and blue in addition to green in my 

 first examination, and later I discovered a trace of red. Ap- 

 parently the transmitting power of the solutions changed 

 slightly during the course of the experiments. On this 

 account certain solutions are undesirable for experiments 

 on color vision, for one must be certain of the constancy of 

 the condition of stimulation. It is to be understood, of 

 course, that each of the three filters transmitted, so far as 

 the eye is concerned, only the color named. I consider the 

 red filter perfectly satisfactory, the blue- violet very good, and 

 the green poor Henceforth, in testing color vision in ani- 

 mals, I shall make use of colored glasses as filters, if it is in 

 any way possible to obtain or have manufactured blue, green, 

 and yellow glasses which are as satisfactory as the ruby. 



The apparatus needs no further description, as its other 

 important features were identical with those of the reflected 

 light experiment box. The use of artificial light for the 

 illumination of the electric-boxes made it necessary to con- 

 duct all of the following tests in a dark-room. The method 



1 A Janssen-Hoffman spectroscope was used. 



