46 MICROSCOPIC ILLUSTRATIONS. 



in our lenses, through the necessity of their being 

 cemented together to prevent loss of light. To say 

 nothing of the errors arising from the irrationality of 

 the spectrum — of the difficulty there is in centering and 

 adjusting the lenses — of the minuteness of the scale upon 

 which the whole is constructed, insomuch that the 

 slightest variation in the thickness of a lens (imperceptible 

 to the eye of a workman) is quite enough to alter the. 

 state of the aberrations in finely corrected object-glasses 

 of great angular aperture ; nay, that even the thickness 

 of the film of glass, or mica, used for the purpose of pre- 

 serving an object, may do this — it is not difficult to under- 

 stand that there have been great obstacles to surmount 

 in constructing an achromatic. 



The talented Professor Amici, about the period of his 

 first attempt at achromatics, invented a reflecting micros- 

 cope. This instrument not having any chromatic disper- 

 sion to contend with, and only one surface to be figured, 

 was soon constructed with a considerable angle of aper- 

 ture. Hence its performance so far excelled that of any 

 other, that he was induced to lay aside his refracting 

 microscope at that time. 



When an account of this microscope reached this 

 country, Dr. Goring, in 1824, suggested the idea of 

 working its concave ellipsoidal surface with a shorter 

 focus and a larger angle of aperture. Now, although 

 the working of a metal is in itself more difficult than the 

 forming a surface to a lens, yet the whole thing exhibited 

 so many facilities over the constructing of an achromatic 



