BY ALEXANDER SILVERMAN 99 



adjustment enables the operator to keep the lamp at a constant 

 distance from the object under examination. 



The Shutter. — A shutter, which slips inside the lamp circle, may- 

 be placed under the lamp to cut off the light from one-half of the 

 circle, so as to produce oblique illumination where this is desirable. 

 Where depth without shadows is desired the shutter is unnecessary. 



The Absorjition Disc. — This is a dull black disc for covering 

 highly polished surfaces, so that only the small portion under exam- 

 ination is exposed to the light. 



Photomicroyrafhy . — For photomicrographic work the illuminator 

 is attached as already described, and the camera employed without 

 lenses, except those contained in the objective and ocular. For 

 work done in this laboratory the camera shutter was left wide open. 

 16 and 32 mm. objectives were employed wdth a lOx ocular. As 

 most microscopes are now equipped with vertical illuminators, the 

 tube of such microscopes should be extended about 16 mm. when 

 the vertical illuminator is removed and the new one attached. 

 It is also desirable to use a Davis shutter in conjunction with the 

 objective. Hammer ortho extra rapid plates were exposed for from 

 10 to 40 seconds, depending on the nature of the object photographed. 



Low Power Work. — Excellent results have been obtained with 

 low power objectives from 60 mm. to 16 mm. By using the stage 

 adapter for 32 mm. and less powerful objectives, it is possible to 

 place the lamp about one-quarter of an inch from the object and 

 obtain beautiful effects. This is of advantage also with the double 

 objective binocular microscope. 



High Power Work. — The illuminator has proven satisfactory for 

 oil-immersion work with a 1.8 mm. objective and 15 x ocular (1,425 

 diameters). The markings on diatoms and structure of fine-grained 

 alloys show clearly. 



liedf of the Lamp. — To allay any fear concerning the heat 

 radiated or conducted from the lamp, the writer begs to state that 

 in his laboratory the lamp was attached to various objectives and 

 run continuously at 100 per cent, over-voltage for more than half 

 an hour without doing any harm to the objectives. Dr. E. M. 

 Chamot, of Cornell University, conducted an independent series of 

 experiments in which he drilled a hole in the side of the objective, 

 inserting a small pyrometer tube between the lenses. He burned 

 <ihe lamp continuously over ' long periods, and pronounced it 

 harmless. 



Advantages. — The new illuminator, when used for the examina- 

 tion of opaque objects and others which may be viewed by reflected 

 light, shows a greater wealth of detail than is obtainable by older 

 methods. 



