3 6 



PHYSIOLOGICAL PHYSICS. [Chap, xxvin. 



Amici, of Modena (1827), there was shown a method 

 for rendering the object-glass of a highly magnifying 

 microscope completely achromatic. The combination 

 consists of three pairs of lenses, each pair being made 

 of a doubly convex lens of crown glass, cemented by 

 means of Canada balsam (whose refractive index is 

 the same as that of crown glass) to a plano-concave 

 lens. These lenses are placed close to one another, 

 the plane surface being towards the object, and are so 

 arranged that one lens corrects the errors of the other. 

 Fig. 162 shows this combination, in position in the 



supporting tuoe. With corrected 

 lenses also the angular aperture is 

 increased. The angular aperture 

 is represented by the side part of 

 Fig. 162 by the angle bfb'. This is 

 the angle formed by the extreme rays 

 which are able to pass through the 

 system of lenses. Thus in the figure, 



Fig. 162. - Achro- tne ravs f a /' are too oblique to 

 matic combina- pass through the three pairs of 



tion and Angle J i ?i 7 /., / * 



of Aperture. lenses, but the rays jb fb pass, and 

 it is between them the angle of 

 aperture is contained. It is evident, of course, that 

 the more rays that pass through the system of lenses 

 the better illuminated will the object appear to be, 

 and the fewer the rays the dimmer the object. So 

 that, from this point of view, any method which in- 

 creases the angular aperture, and thus increases the 

 illumination, is an improvement. Yet it is to be 

 noted that the more that oblique rays are caused to 

 pass through the system, the greater is the difficulty of 

 correcting for spherical aberration; and, even when 

 the correction is complete, the narrower is the 

 border-land between clear definition and blurring of 

 the object. 



Another point remains to be noted about the 



