778 RECORD OF CURRENT RESEARCHES RELATING TO 



the objective when used to form an image in the field of the eye-piece 

 of the Microscope. 



So much for the air angle; for balsam, or what we will here 

 consider as the same thing, angle in glass, the slide g is replaced by 

 another (Fig, 13) of the same thickness, but with a small bull's-eye — 

 say • 25 inch radius — cemented to it, and of such thickness that its 

 centre of curvature is in the front surface of the slide ; fine lines are 

 also ruled on this slide passing through the centre of curvature of the 

 lens. When the bull's-eye is properly adjusted, it will make no 

 difiierence in the distinctness with which the images of external 

 objects are exhibited when using the objective and eye-lens / as a 

 little telescope, whether the glass slide and bull's-eye are in position, 

 or whether they are removed, the rays pass through the slide and 

 bull's-eye, emerging without refraction at the convex surface ; they 

 will emerge indeed at a much smaller angle, as shown in Fig. 14, 

 from the refraction at the front surface ; and in this refraction nearly 

 all the rays which give the exaggerated angle disappear, and the 

 angle of the emergent rays is the true angle in glass, from which the 

 true air angle may be computed. To avoid this computation, Zeiss 

 constructs the apertometer with another scale, uj^on which an arc 

 of 82° corresponds to 180=, and one of 77-5° to 144°, &c. 



A Spencer |^ inch, when the systems were closed, and it was 

 adjusted on the cross lines in the centre of curvature of the hemi- 

 spherical lens, transmitted rays making an angle of 77*5°. The 

 natural sine of half this angle is • 6259, and this, multiplied by 1 • 52, 

 assumed as the index of refraction of glass, gives 'OSIS, the natural 

 sine of 72*05^, twice which, or 144' 1°, is the air angle. Measvired 

 directly, using the glass slide g, and the lines as before described, 

 144° was obtained. Measured on the sector in the old way, there 

 was no difficulty in getting 179^ before the light disappeared. 



It will be understood that for immersion angle, or, as it is gene- 

 rally called, " balsam angle," all that is necessary is to introduce a 

 di'op of fluid (theoretically, this should have the same refractive index 

 as the glass) between the objective front and the glass slide, and to 

 re-focus on the lines. The ^ inch, at the same closed point which 

 gave only 77 • 5° from air into glass, will give 87° with glycerine inter- 

 posed. There is very little difiiculty in observing the true angle here ; 

 the proper point is when the circle of light, which is beautifully 

 shown when the tissue paper covers the open end of the tube h, is 

 dichotomized, and the slightest movement will cause it to disappear. 

 This action is even more prompt with the fluid interposed than when 

 air intervenes. 



To convert the instrument substantially into Abbe's apertometer, 

 it is only necessary to keep the slide and the bull's eye in place, focus 

 on the lines, and then, putting on the cap with the small eye-hole 

 at b, look in there instead of at /. Placing a sheet of paper in front of 

 the graduated sector, and allowing the light to shine through it, we 

 shall get a distinct picture of the end of the tube /, and see a little 

 spot of light (the eye-hole) in the centre. The lines ruled on the 

 glass may interfere with perfect definition in the middle of the field, 



