JAN0ABY 22, 1915] 



SCIENCE 



141 



the character of the work done in the experi- 

 ment stations. 



Samuel Bradford Doten, 

 Director Nevada Agricultural 



Experiment Station 

 Univeesity op Nevada, 

 October 19, 1914 



Into the upper end of this tube is fitted a 

 wooden plug, X, bored to take a small screw 



SPECIAL ASTICLES 

 A DEVICE FOR PROJECTING A SMALL SPOT OF 

 LIGHT SUITABLE FOR EXPLORING PHOTO- 

 SENSITIVE AREAS ^ 



In experimental work on light reactions the 

 question of the precise location and extent of 

 the photosensitive areas frequently presents it- 

 self. If the organism under observation hap- 

 pens to be small, or if minute sensitive ele- 

 ments are scattered in various parts of the 

 integument, the problem has its difficulties. 

 One of the obvious methods of attack is to 

 explore the animal with a spot of light. To be 

 of practical value for this sort of work the 

 light spot must be small, clearly defined, and 

 without halo, and it should be possible to direct 

 it with the utmost ease and precision. Vari- 

 ous devices have been employed for this pur- 

 pose, none of which has proved entirely satis- 

 factory. The use of a " pinhole " aperture does 

 not give a sharply defined spot of light at a 

 convenient working distance. An elaborate 

 system of collecting and focusing lenses is 

 expensive and is very likely to be cumbersome 

 to handle. After trying various schemes, I 

 found that by inserting a small tungsten bulb 

 into a microscope in place of the ocular and 

 projecting the rays through the objective, a 

 spot of light could be produced which fulfilled 

 the requirements admirably.^ 



The accompanying figure shows the details 

 of the apparatus. A piece of brass tubing, P, 

 is turned to fit into the draw-tube of the 

 microscope in place of the ocular, a collar being 

 left on it to prevent it from sliding in too far. 



1 rrom the Museum of Comparative Zoology, 

 Harvard University, and the Anatomical Labora- 

 tory of the School of Medicine, Western Beserve 

 University. 



2 The idea of utilizing the lenses of a microscope 

 was suggested by Dr. Clark of the Physics depart- 

 ment of Harvard University. 



Fig. 1. A Device for Projecting a Small Spot 

 of Light Suitable for Exploring Photosensitive 

 Areas. TV, wires from batteries to light; X, 

 wooden plug fitted into the tube P, and bored to 

 receive socket ; S, screw socket for light ; L, 2i-volt 

 tungsten ' ' flash-light ' ' bulb ; D, metal diaphragm 

 with small circular aperture; Z, cork collars hold- 

 ing diaphragm in place; G, diaphragm to cut out 

 reflection from inside of tube, P; P, brass tube 

 fitting into microscope in place of the ocular; T, 

 draw- tube of microscope; M, barrel of microscope; 

 P,, construction lines indicating formation of the 

 image, I; 0, ocular; /, inverted and reduced image 

 of aperture in diaphragm, D. 



