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WILLIAM BRODBECK HERMS 



cylindrical lens, i.e., at the position of the centre of the stage. 

 Using diaphragms with triangular openings and equal bases, but 

 with different altitudes, light could be obtained which either graded 

 off rapidly or gradually into darkness; but in case an ungraded 

 field was desired, a diaphragm with a rectangular opening was 

 substituted. 



Fig. 2 illustrates in plan an apparatus which proved quite 

 useful in producing a pencil of light both intense and well defined. 

 A 77.4 cp. Nernst light was used at a distance of 50 cm., and the 



Fig. 2 Plan of light pencil apparatus. A, chamber containing a Nernst lamp 

 B {17 A cp.); G. and H, diaphragms: C, a darkened cylinder with diaphragm /;D, 

 reading glass; E, a second blackened cylinder with a pinhole diaphragm J; K, light- 

 pencil; L, incandescent lamp used to start the larvae in the direction of the light- 

 pencil K, and controlled by the switch M; F, a sheet of smooth slate used as a stage 

 for the larvae to creep upon. 



light pencil was from 1 mm. to 2 mm. in width at the point of 

 experimentation, varying with the path of the traveling larva. 

 There was so little diffusion that it was quite impossible to see the 

 moving individual until it came into the light at right angles to 

 its own path. In order to start the larva in the proper direction, 

 the light L (fig. 2). controlled by the switch M was turned on for 

 a few moments and again turned off as soon as the larva began 

 moving in the desired direction. By using a smaller stage and 

 suspending it, the light pencil could be played on the larva at will. 



