514 COMPARATIVE ELECTRO-PHYSIOLOGY 



vertically below the mirror lies in the fact that a lighted 

 candle may be placed in the dark room without spoiling 

 the record by diffuse illumination. The only way in which 

 such diffuse light could now find access to the plate would 

 be by reflection from the ceiling. But if the ceiling of the 

 experimental room is blackened, or a black cover placed over 

 the nerve-chamber at a certain height, even this possibility is 

 eliminated, The advantage which the observer enjoys, when, 

 instead of groping in semi-darkness, he can work in a fairly 

 well-lighted room is obvious. By making the arm of the 

 lever to which the nerve is attached sufficiently short, and by 

 placing the recording plate sufficiently far away, a wide 

 range of magnification, from several hundreds to several 

 thousands, may be obtained. It may sometimes be desirable 

 to subject the nerve to a certain amount of tension, and 

 this is secured by placing a small weight on the arm of 

 the lever. With high magnification, due adjustment, which 

 is very troublesome, lies in bringing the spot of light con- 

 veniently over the recording plate. This difficulty is obviated, 

 however, by means of a fine micrometer screw S which moves 

 the whole nerve-chamber up or down, in relation to the 

 Optical Lever. The adjustment of this screw in a right- 

 handed manner then moves the spot of light in one direction, 

 say to the left, while its left-handed rotation moves it to the 

 right. This movement can be made very fine, and the spot 

 adjusted to any part of the photographic field. 



It remains to deal with the possible disturbances inci- 

 dental to the high magnification employed. Apprehension, 

 in this matter, is often more fanciful than real. Disturbances 

 might no doubt occur, however, when proper conditions are 

 not secured for the experiment. If the nerve-chamber, for 

 example, be supported on a different stand from that of the 

 Optical Lever, then the slightest tremor of the common 

 pedestal would result in relative movements of the two 

 supports, causing constant disturbance of the spot of light. 

 Under these conditions, heavy stone pedestals, erected on 

 steady foundations, afford no security against the ground- 



