6 CARL VON HESS 



I began with measurements of the normal human eye in order 

 to determine the average pupillomotor irritant value of the 

 various colored lights. Further measurements of relatively blue- 

 seeing, red-green blind (so-called red-blind), showed, as may be 

 seen in the table, that a very slight irritant value of red, and a 

 hardly perceptible variation from the normal motor-irritant value 

 of blue, are characteristic of this disturbance of the sense of 

 sight. For the sake of brevity I shall limit myself in the fol- 

 lowing to- the discussion of the red and blue values, these being 

 of the greatest importance to us. In two cases of totally color- 

 blind which I have repeatedly examined, red proved to have a 

 very slight motor-reactive value (<0.6), blue, a comparatively 

 high value of 9-11.8% (as compared to 1.5-2.5% in the normal 

 eye). These are the three principal kinds of pupil reactions 

 which occur among normal and color-blind human beings and 

 with these we must compare the motor reactive values found 

 among the different animals. 



For the day bird, the sensitive value of red is like our own; 

 this corresponds to the fact which I had already discovered by 

 another method, that day birds in most cases see red lights 

 nearly or quite as we see them. The relatively small values 

 of blue, — they are the smallest which I have met with in the 

 animal series — correspond to another fact which I had dis- 

 covered, namely, that day birds in consequence of red and 

 yellow oil globules located in front of the light receiving ap- 

 paratus, are relatively blue blind. 



With the help of the apparatus I was enabled, among other 

 things, to answer the following question, which I raised some 

 time ago. The beautiful blue of the feathers of many birds is 

 interpreted by almost all zoologists as decorative color for the 

 attraction of the other sex: this interpretation assumes that 

 these birds see blue as we see it, that therefore the oil drops 

 do not exist. For if these drops are found in the eyes of these 

 birds as they are found in the hen and the dove, then a blue 

 which seems to us gorgeous must look to them blue-gray or 

 colorless gray. So far I have had no opportunity to examine 

 such birds with the spectrum according to the method described ; 

 but a short time ago I examined the movements of the pupil 

 of the Butterfly-finch (Mariposa phoenicotis) with the new ap- 

 paratus ; the motor values are the same as for chicken and dove ; 



