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III. — On the Angular Aperture of Immersion Objectives, 

 By Egbert B. Tolles, of Boston, U.S. 



Plate CII.— A. 



Fig. 1 represents a section of two hemispherical lenses balsam- 

 cemented, with a diatom or other small object at the centre, together 

 constituting a nearly homogeneous transparent globe. 



Fig. 2. The same, represented as (in section) the front lens of 

 an immersion objective, i. e. as to the part a, while the portion a' 

 corresponds to the last (front) lens of an immersion condenser, both 

 much exaggerated. 



In Fig. 2, rays are traced as immergent and emergent at a per- 

 pendicular incidence, and therefore without any bending at either 

 surface. 



The case is thus completely simplified, and the fact is evident 

 enough that the rays traversing the balsam-mounted object and 

 emerging at the upper surface of the front lens a, have materially 

 more than 82° maximum angle. In Fig. 2, the courses of extreme 

 rays for 90°, 120°, and 170° respectively are traced. 



In each case the real angle of the immersion objective would be 

 the same as the angle of the appropriately applied systems above 

 taken separate from the front lens measured as and constituting a 

 dry objective, and (for the sake of simplicity of the case) adjusted 

 for a drif object. 



An objective, such as is above indicated, inclusive of the front 

 hemisphere a, of course could not, as adjusted, work dry, or only so 

 far as actual contact of the object with the plane surface should 

 happen. 



On the contrary, the objective used in the experiments described 

 in my communication to this Journal, July, 1871, did work as a 

 dry objective, and of 170° incident pencil, but by construction was 

 limited to about 220° transmitted pencil when the first plane surface 

 was ehminated, or nearly so, by water in the interspace. 



The above diagrams and comments are given, not from actual 

 trial-proof, but as an illustration too clear, perhaps, to need the 

 demonstration of experiment. 



Let this be added, however, — " No one will have the hardihood 

 to " deny that an object homogeneously cemented centrally between 

 the hemispherical lenses a a! can be seen (looking through the 

 sphere diametrically with a simple magnifier) from every jioint of 

 view, thus giving " image-forming rays." 



The case is totally and most obviously applicable to that of the 

 ordinary balsam-mounted microscope object for an aperture far 

 above 82° of angular pencil actually traversing the object and made 

 available in the view to the eye of the observer. For obtainment 



