THE MICROSCOPE. 



To render the diagram more easy of comprehension, we have 

 not here attempted to represent the several distances in their 

 proper proportions. 



The compound lenses, of which object-pieces consist, are 

 generally, as represented in the figure, plane on the sides presented 

 towards the object. This is attended, among other advantages, 

 with that of allowing a larger angle of aperture than could be 

 obtained if the surface presented to the rays diverging from the 

 object were convex. 



The extreme rays diverging from each point of the object/ fall 

 upon the surface of the object-glass with a greater and greater 

 obliquity as they approach its borders, and since there is an 

 obliquity so extreme that the chief part of the rays would not 

 enter the glass at all, but would be reflected from it, the angle of 

 aperture must necessarily be confined within such limits, that the 

 rays falling from the borders of the lens will not be so oblique as to 

 come under this condition. If the surface of the object-glass pre- 

 sented to the object were convex, it is evident that the rays diverging 

 from an object at a given distance from it would fall upon its borders 

 with greater obliquity than if it were plane, and, consequently, 

 such an object-glass would allow of a less angle of aperture than 

 a plano-convex one with its plane side towards the object. 



Improvements have recently been made in object-glasses, by 

 which angles of aperture have been obtained so great, as not to 

 admit even of a plane surface being presented to the diverging 

 pencil, and it has accordingly been found necessary, in such cases, 

 to give the object-glasses the meniscus form (Optical Images, 25), 

 the concave side being presented to the object. By this expedient 

 angles of aperture have been obtained so great as 170. If the 

 surface of the object-glass presented to the object were plane, the 

 extreme rays of the central pencils, with such an angle of aper- 

 ture, would fall upon the surface of the lens with obliquities of 

 not more than 5, and the obliquities of the extreme rays of 

 the lateral pencils would be even less. Under sttch circum-^ 

 stances, the chief part of the rays near the borders of the 

 lens would be reflected, and, consequently, its virtual would be 

 less than its apparent angle of aperture. It is questioned by some 

 microscopists that even with the expedient of a concave external 

 surface, a practically available angle of aperture so great as 170 

 can be obtained. 



The three achromatic lenses here described being mounted, so 

 that their axes shall be precisely in the same straight line, con- 

 stitute what is generally called an OBJECT-GLASS, but which, 

 perhaps, might with more convenience and propriety be denomi- 

 nated an OBJECT -PIECE. 

 22 



