52 PRACTICAL MICROSCOPY. 



The J-inch objective, yielding a power of 200 diameters 

 with the A eye-piece, is a most useful glass to the student, 

 when possessing an air angle of about 85. It is con- 

 structed of a triple back lens, a double 

 middle, and a single front. Messrs. Ross's 

 system is believed to differ from this, inas- 

 much as their objectives, from the J-inch 

 upwards, are supposed to be constructed on 

 Mr. Wenham's new formula, in which the 



"" flint concave of a triple middle is made to 



FIG. 30. 



correct the aberrations of the anterior and 

 posterior crown lenses. An enlarged section of this formula 

 is shown in Fig. 31. 



Quarter-inch objectives may be obtained of as high an 

 air angle as 140, very good for the experienced micro- 



scopist but of limited use to the 



student, as such glasses focus very 

 close to the object. 



All objectives possessing a 

 numerical aperture of more than 

 / "v. 0*42 (50) should be furnished with 



| a screw collar to adjust the lenses 



' for varying thicknesses of covering 



FIG. 31. , , . . , , 



glass, and this is specially needed 



on Mr. Wenham's new formula. It is often a very tedious 

 operation to find the exact adjustment requisite ; but it may 

 be approximately performed in the following manner : 



Set the collar at zero, and focus the glass upon the 

 object ; next turn the collar until the dust upon the cover- 

 glass is in focus, when an approximate correction will have 

 been applied ; if the observer again focusses by means of 

 the fine adjustment, the object will be found more sharply 

 defined than before. This may only be considered a rough 

 method. The best way is to work at some well-known 



