INFLUENCE OF THE COVER-GLASS. 



65 



lens has a repellent influence upon those of the final virtual 

 image. Attraction and repulsion decrease, of course, from with- 

 out inwards, and in the centre become nil. Curvature of the 

 image-surface is therefore due to spherical aberration, and is 

 more or less apparent by turning the convex side of the aplanatic 

 system upwards or downwards. 



YI. 



INFLUENCE OF THE COVEK-GLASS. 



A GLANCE at Fig. 25 will show that two rays which proceed from 

 a point a, after their passage through a medium bounded by 

 parallel plane surfaces (m n and p q\ will appear to have come 

 from a point a, more or less distant from a. Tf both rays are 

 equally inclined to the refracting surfaces, the two points a and of 



FIG. 25. 



FIG. 26. 



will lie in a straight line perpendicular to the surfaces ; if, on the 

 other hand, they are unequally inclined, the line joining them will 

 form an oblique angle with the perpendicular. 



A pencil of light, whose rays diverge from, the object-point a, 

 will consequently be no longer homocentric on emerging from the 

 cover-glass, but will consist of an infinite number of superposed 

 cones, whose apices vary with the obliquity of the incident cone 

 in relation to the cover-glass. The real object-point is therefore 

 represented virtually as a line, whose length evidently varies with 

 the thickness of the cover-glass and the . angle of aperture of the 

 objective. 



