QUANTITATIVE DETERMINATION OF LIGHT REACTIONS 221 



Fig. 1 Diagram of apparatus used to produce differential bilataral light 

 stimulation. G, five 220-volt Nernst glowers; M and M', mirrors;/ and/' central, 

 point of mirrors; 0, center of observation stage; dotted lines, central ray of beam 

 of light from the glowers reflected to by the mirrors; d and d', screens with rec- 

 tangular openings ; s and s', light shields ; a and b,2c.p. orienting light with screens. 



The central ray of light from the apertures fell on the centers of 

 the vertical mirror^ilf and M', which were cut from the best French 

 glass, and, to insure uniformity, from the same piece. These mirrors 

 were placed at such an angle that they reflected the central ray of 

 the beams Gf and Gf to the observation point directly opposite 

 the glowers. It will be seen that the apparatus was so constructed 

 that the courses taken by the light over the two paths from G to 

 were equal in length and symmetrical in position. Further- 

 more, the central rays impinging at were in the same straight 

 line but came from opposite directions. The two beams of light 

 reaching this central point should be of equal intensity, and care- 

 ful photometer tests showed them to be so. By using a single 

 source of light for both beams, the uncertainty due to the pos- 

 sible fluctuations in two separate sources of light was eliminated. 



THE JOUHN.^LOF EXPERI.ME.VTAL ZOOLOGY, VOL. 17, NO. 2 



