84 THE MICROSCOPE. 



aberration in front would affect a pencil of 140*. Sub. 

 stituting a homogeneous medium, the same pencil, 

 contracted to the equivalent angle in that medium of 

 112, will be admitted to the front lens without any 

 aberration, and may be made to emerge from the curved 

 surface also without any detrimental aberration, but 

 contracted to an angle varying from 70 to 90. The first 

 considerable spherical aberration of the pencil then 

 occurs at the anterior surface of the second lens, where 

 the maximum obliquity of the rays is already con- 

 siderably diminished. 



Figs. 44 and 44# will serve to further elucidate this. 

 If the objective of 140 works with water (fig. 44), 

 there would be a cone of rays extending up to 70 on 



FIG. 44. FIG. 44a. 



both sides of the axis, and this large cone would be 

 submitted to spherical aberration at the front surface a. 

 But with homogeneous immersion (fig. 440) the whole 

 cone of 112 is admitted to the front lens without any 

 aberration, there being no refraction at the plane sur- 

 face ; and as the spherical surface of the front lens is 

 without notable spherical aberration, the incident pencil 

 is brought from the focus P to the conjugate focus /, 

 and contracted to an angle of divergence of 70 90 

 without having undergone any spherical aberration 

 at all. 



Thus the problem of correcting a very wide-angled 

 objective is reduced by the homogeneous- immersion 



