CH. 7] 



MICROSCOPE AND ACCESSORIES 



2 35. Aperture of Objectives. The angular aperture or angle of aperture 

 of an objective is the " angle contained, in each case, between the most diverg 

 ing of the rays issuing from the axial point of an object [z. e. , a point in the 

 object situated on the optic axis of the microscope] , that can enter the objec- 

 tive and take part in the formation of an image." (Carpenter. ) 



In general the angle increases with the size of the lenses forming the 

 objective and the shortness of the equivalent focal distance ( 18). If all 



FIG. 31. The tube of a micro- 

 scope with ocular micrometer and 

 nose piece in position to show that in 

 measuring tube-length one must 

 measure from the eye lens to the place 

 where the objective is attached. 

 (Zeiss' Catalog.} 



objectives were dry or all water or all homogeneous immersion a comparison 

 of the angular aperture would give'one a good idea of the relative number of 

 image forming rays transmitted by different objectives ; but as some are dry, 

 others water and still others homogeneous immersion, one can see at a glance 

 that, other things being equal, the dry objective (Fig 33) receives lesg light 



FIG. 32. Diagram illustrating the angular aper- 

 ture of a microscopic objective. Only the front lens of 

 the objective is shown. 



A.ris. Theprincipal optic a.vis of the objective. 



B A, B C, the most divergent rays that can enter 

 the objective, they mark the angular aperture. A B D 

 or C B D half the angular aperture. This is designated 

 by u in making Numerical Aperture computations. 

 Seethe table, (\ 39). 



than the water immersion, and the water immersion (Fig. 34) less than the 

 homogeneous immersion (Fig. 35). In order to render comparison accurate 

 between different kinds of objectives, Professor Abbe takes into consideration 



