The Sense of Sight : Color Vision 157 



upon the brightness must be taken into account. The two 

 red niters were alike in their influence upon the light which 

 entered them, for they were precisely alike in construction, 

 and the same was true of the two blue-violet filters. The 

 same kind of ruby glass was placed in each of the former, 

 and a portion of the same solution of copper ammonium 

 sulphate was put into each of the filter boxes for the latter. 

 But it is difficult to say what relation the diminution in 

 brightness caused by a red filter bore to that caused by a 

 blue-violet or a green filter. My only means of comparison 

 was my eye, and as subjective measurement was unsatis- 

 factory for the purposes of the experiment, no attempt was 

 made to equalize the amounts of brightness reduction caused 

 by the several filters. So far as the value of the tests them- 

 selves, as indications of the condition of color vision in the 

 dancer is concerned, I have no apology for this lack of measure- 

 ment, but I do regret my inability to give that accurate ob- 

 jective statement of brightness values which would enable an- 

 other experimenter with ease and certainty to repeat my tests. 

 The nearest approach that it is possible for me to make to 

 such an objective measurement is a statement of the com- 

 position and thickness of the filters and of the candle-meter 

 value of the light when it entered the filter. The distance 

 from this point to the entrance to the electric-box was 20 cm. 

 To sum up and state clearly the method of defining the 

 brightness of the light in the following experiments: the 

 candle-meter value of each light by which an electric-box 

 was illuminated, as determined by the use of a Lummer- 

 Brodhun photometer and measurements of the distance of 

 the source of light from the filter, is given in connection with 

 each of the experiments. This brightness value less the 

 diminution caused by the passage of the light through a 

 filter, which has been denned as to composition and thick- 



