MAGNIFICATION 



107 



SECTION II. THE OPTICAL PARTS 

 OF THE MICROSCOPE. 



The great difficulty in selecting a 

 microscope is the choice of the lenses. 

 For ordinary work two eyepieces, and 

 four objectives including a T Vin. oil- 

 immersion lens, are all that is neces- 

 sary. 1 



A T V m - or T V m - homogeneous im- 

 mersion lens may be of use occasionally. 



In addition to the microscope and 

 its lenses, it is convenient to have a 

 camera lucida and a stage and ocular 

 micrometer. 



A. The objectives. 2 



The use of a microscope is to magnify 

 the details of an object, so that those 

 invisible to the naked eye may with its 

 aid be easily seen. The essential re- 

 quisites then in good lenses are definition 

 and magnification. 



1. Magnification. 



The apparent linear size of an object AB 

 (fig. 99) varies inversely as its distance BK 

 from the eye of the observer, and depends 

 upon the tangent of the visual angle a which it subtends at the nodal point K 

 of the eye. T> \ 



tan a = tan BK A = ^^' 

 JVr> 



Now, let B'K denote the least distance of distinct vision (10 inches), and let it be 

 denoted by I. 



FIG. 98. A microscope. 



Then the greatest apparent size of BA to the unaided eye is when it is in the 

 position B'A', and its apparent size is then 



, B'A' B'A' 

 = KB' = ^T' 



It follows from this that the larger the angle subtended by the object at K, the 

 larger will the object appear to be. And a microscope is nothing more than an 

 instrument with which to increase the size of this angle. 



1 In French makes, and also in Reichert's and Leitz' lenses a No. I. or No. II. and a 

 No. III. eyepiece, and a 2, 6, and 8 or 9 dry objective. In Zeiss' list the corresponding 

 objectives are AA, DD, and E in the dry series, and eyepieces, 2, 4 and 8. 



[ 2 The remainder of this section, dealing with the theory of the microscope, has been 

 rewritten and considerably extended. H. J. H.] 



