MAGNIFYING POWERS. 173 



be 80 much disturbed aa where the angles are great, and 

 where, consequently, the aberrations increase rapidly. 



"When an object-glass haa its aberrations balanced for 

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

 object by transmitted light, the correction will remain; but 

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

 with glass or mica, an aberration will arise from these circum- 

 stances which will disturb the previous correction, and, con- 

 sequently, deteriorate the definition, and this eflfect will be 

 more obvious with the increase of the distance between the 

 object and the object-glass. The aberration produced with 

 diverging rays by a piece of flat and parallel glass, such as 

 would be used for covering an object, is represented at fig. 121, 



rig. 121. 



where G G G G is the refracting medium, or a piece of glass 

 covering the object, O, and O P the axis of the pencil, perpen- 

 dicular to the flat surfaces, O T a ray near the axis, and 

 O T' the extreme ray of the pencil incident on the under 

 surface of the glass : then T K, T E' will be the directions of 

 the rays in the mediiun, and E, E R' E' those of the emergent 

 rays. Now, if the course of these rays is continued as by the 

 dotted lines, they will be found to intersect the a^s at dif- 

 ferent distances, X and Y, from the surface of the glass ; and 

 the distance, X Y, is the aberration produced by the medirim 



