62 LIGHT AND THE BEHAVIOR OF ORGANISMS 



and stage are at the conjugate focal points of the lens, and 

 therefore at equal distances (50 cm.) from it. The plano- 

 convex cylindrical lens used is 25 cm. long, 10 cm. wide and 

 has a radius of curvature of 12.5 cm. 



A cylindrical lens will not form a single definite 

 image of an object, but rather a series of images, since by 

 means of it light is focused only in reference to one plane. 

 If, then, the object, e.g., a Nernst glower, is placed at one 

 of the conjugate focal points so that the distance from the 

 lens to the glower is equal to that from the lens to the 

 image, and the glower is so arranged that it is perpendicular 

 to the axis of the lens, the image will not consist of a narrow 

 band of light as large as a glower, which would be true if 

 the segment of a sphere were used as the lens, but it will 

 consist of a comparatively large field of light, the length of 

 which is proportional to the functional length of the lens, 

 while the width is equal to the length of the glower, regard- 

 less of the functional width of the lens (see Fig. 4). But 

 since the amount of light which passes through the lens is 

 directly proportional to the functional width of the lens 

 and the width of the field is constant, it is clear that the 

 intensity of light in the field, if we disregard the amount of 

 light absorbed by the lens, must also be theoretically pro- 

 portional to its functional width. Direct measurements of 

 the light intensity with different functional widths of the 

 lens proved this to be true within the limits of error. If, 

 then, the lens be covered with an opaque screen containing 

 a triangular opening, the base of which is parallel with the 

 minor axis of the lens as represented in Fig. 4, there will 

 result a rectangular field of light in which the intensity 

 gradually diminishes from the end produced by light which 

 passes through the base of the triangular opening to the 

 opposite end, where theoretically it fades into darkness. 

 Practically, however, it was found to be impossible to cut 

 the apex of the triangular opening so as to prevent an 

 apparent line at the end of least intensity. Since the light 

 intensity of the field is proportional to the functional width 



