32 



THE MICBOSCOPE. 



" When an object-glass has its aberrations balanced for view- 

 ing an opaque object, and it is required to examine that object 

 by transmitted light, the correction will remain; but if it is 

 necessary to immerse the object in a fluid, or to cover it with 

 glass or talc, an aberration will arise from these circumstances, 

 which will disturb the previous correction, and consequently 

 deteriorate the definition; and this effect will be more obvious 

 with the increase of the distance between the object and the 

 object-glass. 



IP /E E , 



R> 



Fig. 17. 



"The aberration produced with diverging rays by a piece of 

 flat and parallel glass, such as would be used -J or covering an 

 object, is represented at Fig. 17, where G G G G is the refract- 

 ing medium, or piece of glass covering the object O; O P, the 

 axis of the pencil, perpendicular to the flat surfaces; O T, a 

 ray near the axis; and O T', the extreme ray of the pencil inci- 

 dent on the under surface of the glass; then T E, T' E', will be 

 the directions of the rays in the medium, and E E, E' E', those 

 of the emergent rays. Now if the course of these rays is con- 

 tinued, as by the dotted lines, they will be found to intersect 

 the axis at different distances, X and Y, from the surface of the 

 glass; and the distance X Y is the aberration produced by the 

 medium which, as before stated, interferes with the previously 

 balanced aberrations of the several lenses composing the object- 



