MEDIUM-POWER PHOTO-MICROGRAPHY 89 



Care must be taken both in pulling out and pushing in the draw tube not to alter the 

 adjustment of the objective under examination by accidentally shifting the main 

 tube. 



The indices are now adjusted, taking care they lie close to the glass plate until 

 their sharp points just touch the periphery of the luminous circle seen on looking 

 down the eye-piece. Their position found, the readings of the upper edges, which lie 

 in the same vertical plane as the points, are read from one of the two scales on the 

 plate. The half of the sum of the two readings on the outermost scale — that nearest 

 the edge — will give the measured value of the N. A. of the objective under examina- 

 tion. Likewise the sum of the two readings on the inner scale will give the value of 

 the angular aperture in air. 



The illumination must be shifted from right to left or up and down, so that the 

 light falls horizontally upon the edge of the glass. 



A note should here be added to which attention is not sufficiently made in the 

 Instructions we think. 



If the apertometer be used on ^oi(;-power objectives or on condensers such as a half- 

 inch, hut with high N. A., owing to the size of the back lens having to be so large to 

 bear its necessary ratio to the front one which is itself also large ; so the auxiliary 

 lens (which we think too small) may not be of sufficient angle to give the maximum 

 N. A. of the objective under examination. Also, and this is commoner still, it is not 

 at all improbable the ordinary eye-piece, whether achromatic or apochromatic, may 

 not command sufficient field of view either ; so between the two troubles a false N. A. 

 may be placed upon the low power objective or condenser. This actually happened 

 in our case when testing a condenser of high N. A. and an apochromatic half-inch — 

 both estimations came out too small. The mistake was discovered by applying the 

 next means about to be described for ascertaining the N. A. of an objective which gave 

 us such diffi^rent results. To remedy the evil with the apertometer, let the observer 

 look down the microscope after the first focussing (doing away with auxiliary lens), 

 and regulate the indices without an eye-piece at all. 



Special instructions are given with each instrument, but lengthy as the description 

 must necessarily appear, the apparatus is not difficult to use, the only fault is its 

 expense. Still it is the only plan with immersion objectives. 



With dry powers, how^ever, a simpler method of obtaining the approximate N. A. 

 may be employed, and as it is not to be found in any text-book that we are aware of, 

 we will describe it in as few words as possible. 



Lay upon the table two pieces of white paper, using a black background, with 



M 



