10 'Journal of Co^nparative Neurology and Psychology. 



faint band of red and of green light on the screen. In order to 

 intensify and sharply define the hands on the screen, two small 

 achromatic lenses, L^ and Lr„ are interposed in the pathways of 

 the two beams, B and G respectively. These two lenses are of short 

 focal length (6"). They project sharply defined and enlarged 

 images of the two openings of So npon the screen (marked red 

 and GKEEN in the diagram). These images are abont 7" in height 

 and 1%" in width. . 



Four reversing mirrors, BM, are used in order to reverse the 

 right and left positions of the two beams. These mirrors revolve 

 in a vertical plane. They are mounted in bearings in such a way 

 ■that the small weights, Wgt, pull them back to the 45° position 

 whenever the cords, CBM, are slackened, as in the diagram. These 

 cords are jointly fastened to a rod at X. A single forward pull 

 upon this rod brings all four mirrors to the 180° position, in which 

 position they no longer intercept the two beams. 



A glance at the apparatus will show that when, e. g., the red is 

 on the left, the reversing mirrors have to intercept the beam ; wdien 

 it is on the right, they no longer intercept it and the beam is 

 reflected directly from the mirror, M^, to the screen. It is clear 

 from this that the absolute intensity of the two bands is slightly 

 less in the "reversed position" than in the normal. But this reduc- 

 tion occurs in both bands equally.^ In order to compensate for 

 this reduction, a vertical sliding bar, SB, is placed in the pathway 

 of the beams. 1" x 2" Avindows are cut in this bar at the points 

 where the beams impinge upon it. One half of each window is 



^\11 the mirroi-s in the sj-stem are silvered on the anterior surface. They 

 are kept highly polished by the use of "jewelers' rouge." When not in use. 

 they are kept covered with silk handkerchiefs. The absorption of the light 

 consequently is kept constant and at a minimum. 



[Since the mirrors used in the apparatus are a source of a great deal of 

 trouble and care and since their use causes a certain variation in the absolute 

 intensity of the light, effort was made to find a substitute for them. After 

 some experimentation it was found that total reflection prisms could be 

 made to separate the beams, to space them properly, and finally to reverse 

 them. In addition the dark room at Hopkins is large enough to accommodate 

 the apparatus without the use of the mirror. M„ behind the large prism. 

 The whole apparatus is now "self-maintaining" and completely constant so 

 far as the absorption of the light is concerned.] 



