THE MICROSCOPE. 75 



number of lines per inch, or equally small objects, is concerned, will 

 depend entirely upon the angular aperture of the i or fa, or the fa 

 inch objective which is used in this combination. Now, of course, 

 deeper eye pieces may be used, and the image may be magnified up 

 to perhaps 5,000 diameters, but this amplification will show nothing 

 new in the image, it will only give greater distinctness to the details, 

 and lessen the extent of the field, or number of lines in the field. 



But not all objects which are looked at through the microscope 

 contain 150,000 lines to the inch, or other objects of equal size, nor 

 anything approaching it. In medicine, or clinical use, lower power, 

 and lower angular objectives are oftener required than higher. The 

 extreme angular apertures exhibit objects which are of course no 

 wider than they are thick, and hence such objective does not have 

 enough of what is called penetration to properly exhibit such 

 objects. 



The rule is to use an objective with an angular aperture, and 

 power no higher than is required to properly resolve and magnify 

 the object. Using an objective of 1S0 angular aperture on an 

 object having only 300 lines to the inch, would be like using the 

 telescope in the Washington Observatory for an opera glass. It is 

 evident, therefore, that objectives having low magnifying power and 

 angular aperture are necessary in the microscope. Perhaps the fol- 

 lowing list will comprise a battery of objectives which will most 

 satisfactorily cover the whole ground of microscopy: 



A 3 inch of io° angular aperture. 



" 1 



-'0 



" 45 

 " -L " i8o c 



This list of objectives, with a set of eye pieces of two' inch, one 

 inch, one-half inch, and one-third and one-fifth inches, will enable 

 the microscopist to resolve any objects resolvable by objectives, and 

 magnify them as much as there is any necessity for magnifying 

 them. 



The higher power objectives are made with a screw collar 

 adjustment, which, by approximating the elements or lengthening 

 the space between them, adapt the objective to different conditions, 

 which arise from using different fluids for immersion, and for various 

 thicknesses of cover glasses. In such objectives there is no doubt 



