ANATOMICAL TECHNIQUE 465 



A shutter which fits on the front of the photographic camera is 

 most convenient, and an arrangement which will admit of exposures 

 of from three seconds to a hundredth part of a second is desirable. 

 The Thornton-Pickard shutter, of English manufacture, but used 

 a great deal in the United States, has proved itself of value in this 

 connection. When a color screen is used, it may be laid horizon- 

 tally just above the shutter in view of the vertical position of the 

 camera. A special photomicrographic microscope is desirable, 

 although not absolutely necessary, as any ordinary microscope of 

 good size and weight with adequate optical equipment may be 

 employed for photomicrographic work. The microscope is sup- 

 ported on a bench provided with three legs so that it may be always 

 steady. The upright stand to which the camera is attached should 

 likewise have three brass knobs screwed into its base to insure 

 stability. The best source of light is an arc lamp with hand feed. 

 The ordinary projection lantern answers very well" for this pur- 

 pose if it is not too cumbersome. A condenser should be placed 

 in front of the arc to collect the light, and a water chamber should 

 likewise be interposed between the condenser and the microscope 

 to eliminate heat rays. The use of alum in the water chamber to 

 accentuate the exclusion of the heat radiations is unnecessary, and 

 it is inadvisable on account of the evil effects produced on the 

 apparatus by alum. The lantern should be so mounted that it 

 can readily be lowered and raised, and also inclined at any angle. 

 A fixed lantern may be employed, but its utility is much restricted. 

 The carbons are best inclined at an angle of ninety degrees. The 

 light is taken from the end of the positive carbon, which should be 

 in a horizontal position. The current should be direct and not 

 alternating, although the latter may be used with some degree of 

 success. The advantage of the direct current is the freedom from 

 noise and the intense light emanating from the positive carbon. 

 The amperage should be controlled by a rheostat which will per- 

 mit the passage of from seven or eight to fifteen amperes of current. 

 The high amperages are used in the case of dark-colored or opti- 

 cally opaque objects. In the case of photographs under low magni- 

 fication a ground-glass screen may often be inserted with advantage 

 between the mirror of the microscope and the water chamber. 



