COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 45 



we chose as our stimuli six tones which represent more or less widely 

 separated parts of the spectrum, and added to these a purple. The 

 reproduction of these seven tones by means of combinations of gelatines 

 was not always an easy task. But if the supply of gelatines at hand 

 contains a sufficient variety of tones and of saturations, one readily 

 acquires skill in choosing and combining them, and the desired results 

 may be obtained by the exercise of persevering effort. 



We set up a spectroscope in the room where the experiments were 

 conducted and made a spectroscopic examination of each combination 

 of gelatines before it was finally selected as a color stimulus. It is, of 

 course, essential that this examination be made under the same condi- 

 tions as are to be employed in the exploration experiments. The light 

 emitted by the stimulus-lantern was not white, but of a distinctly yellow 

 tone. This yellowness must be taken into account, and must be neutral- 

 ized by means of appropriate blue gelatines, in the reproduction of the 

 color stimuli. It is, then, essential, for an obvious reason, that the 

 source of light which is to be employed in the experiments themselves 

 should also be employed in the spectroscopic examinations. 



Hellpach found that no combination of the gelatines to which he 

 had access gave a satisfactory yellow stimulus. He was therefore led 

 to employ a light filter of another form. This consisted of two cylin- 

 drical cells of glass, which were placed end to end before the stimulus- 

 lantern. The larger cell was filled with a six per cent solution of 

 dichromate of potassium, the smaller with a five per cent solution of 

 deep-green urano-sulphate, which had been obtained by reducing yellow 

 uranyl-sulphate. We are told that the light transmitted through these 

 solutions proved to be exactly what Hellpach required for his experi- 

 ment. Not only was it a good yellow ; it was also monochromatic. "Im 

 Spektroskop ist dann bei Anwendung weissen Lichtes das ganse Spek- 

 truui bis auf die gelbe Na-Linie total ausgeloscht." Phil. Stud., XV, S. 

 530.) One is tempted to wonder why Hellpach employed white light 

 in this spectroscopic examination. The source of light which he chose 

 for his experiments in retinal exploration was not white, but yellow. 

 If, as this statement seems to indicate, Hellpach employed white light 

 in all of his spectroscopic examinations, it is clear that the color stimuli 

 actually employed in his investigation were not submitted to a spectro- 

 scopic examination at all. One is also led to wonder why, since his 

 liquid filter gave him monochromatic light, he did not employ it, with 

 a changed content, of course, in the production of his other color 

 stimuli. He certainly succeeded in no other case, in even approximat- 

 ing monochromatic light. (See his table, /. c., S. 529.) 



