202 American Quarterly Microscopical Journal. 



that for immersion angle, or, as it is generally called, " balsam 

 angle," all that is necessary is to introduce a drop 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 i-8th inch, at the same closed 

 point, which gave only 77°. 5 from air into glass, will give 87° 

 with glycerin interposed. There is very little difficulty in 

 obtaining 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 {b), 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. If the air intervenes in taking the glass 

 angle with long focus glasses, e. g., a 2-3d inch, I find it necessa- 

 ry to use the lines as described for getting the air angle directly; 

 e. g., a 2-3d inch "professional," of Spencer, marked 35°, ac- 

 tually gave with the bull's eye, before the light disappeared, 

 37° as angle in the glass. Computing the corresponding air 

 angle we get nearly 58° I ! Using the lines, however, bringing 

 them to the edge of the illuminated field on ei*.her side, pre- 

 cisely as for the direct measurement of air angle, ^ave but 24°, 

 from which the computed air angle comes out 36°.8-f-; and 

 measured by using the glass slide (g), I obtained 37°. 



These angles would be reduced when the objective was used, 

 as such, on the tube of the microscope, as the focal point would 

 then be somewhat more distant from the front lens; lengthen- 

 ing out the tube b to ten inches, and using an eye-piece for 

 focusing, lessened the angle scarcely a degree, and so the 

 makers, we may conclude, have really marked this objective 

 within its true angle. 



To convert the instrument substantially into Abbe's aper- 

 tometer, 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 eyehole 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 will get a distinct 

 picture of the end of the tube /, and see a little spot of light 

 (the eyehole) in the center; the lines" ruled on the glass may in- 

 terfere with perfect definition in the middle of the field, but 

 when the sector arm is swung around, the little circle of light 

 can be neatly bisected on each side of the field, as shown in 



