222 BRADLEY M. PATTEN 



A horizontal stage of slate 30 by 48 cm., on which the animals 

 were observed, was placed at the same level as the center of the 

 glowers, and in such a position that the observation point fell 

 at its center. At either end of this stage, a 2-candle power 

 incandescent lamp was set up at a distance of 55 cm. from the 

 central point 0. By means of these 'orienting lights,' (fig. 1 

 a and b) a negatively phototactic animal could be started from 

 either end of the observation plate, directly across the field of 

 light from the mirrors. Light screens around the glowers, s, 

 and a larger set of screens around the observation stage, s' , shut 

 out extraneous light and reduced reflection to a minimum. The 

 orienting lights also were screened except for a small horizontal 

 aperture throwing its central ray to 0. 



With the apparatus thus set up, a maggot could be made to 

 move on to the observation stage, -away from one of the orient- 

 ing lights, so that two equally intense beams of light fell on its 

 opposite sides. By reducing the intensity of one of the beams of 

 light, differential bilateral stimulation could be set up, of any 

 desired proportion or intensity. It was important to find a 

 reliable method of reducing one of the beams of light. The 

 episcotister, though very convenient, has been shown to be an 

 unreliable means for the accurate reduction of light intensity 

 (Parker and Patten '12). Several forms of gratings were tried, 

 none of which gave a perfectly uniform field. The difficulty 

 of securing a uniform field, with diaphragms of small enough 

 aperture to give the desired intensity differences, was very great. 

 The most satisfactory means seemed to be to move the observa- 

 tion stage to points on the basal arm of the apparatus where, 



according to the law I oc , v the intensities should be of the de- 



d-, 



sired ratio. Such a method, though cumbersome, gave an ab- 

 solutely uniform field, and the intensity could be figured with 

 great accuracy.- 



- Hyde ('06) has computed very accurately from the law I ixt^ the amount of 



variation when the source of light is a cylinder of the dimensions of a Nernst 

 glower insteadof a point. For tj;ie distances used in this apparatus, the variation 

 is from -(-0.08 per cent to +0.03 per cent. 



