PROGRESS IN ANATOMY AND PHYSIOLOGY 



This undertaking, however, was beset with very great 

 optical difficulties, and for a long time little advance 

 was made upon the work of preceding generations. 

 Two great optical barriers, known technically as spher- 

 ical and chromatic aberration the one -due to a failure 

 of the rays of light to fall all in one plane when focalized 

 through a lens, the other due to the dispersive action of 

 the lens in breaking the white light into prismatic col- 

 ors confronted the makers of microscopic lenses, and 

 seemed all but insuperable. The making of achromatic 

 lenses for telescopes had been accomplished, it is true, 

 by Dolland in the previous century, by the union of 

 lenses of crown glass with those of flint glass, these two 

 materials having different indices of refraction and dis- 

 persion. But, aside from the mechanical difficulties 

 which arise when the lens is of the minute dimensions 

 required for use with the microscope, other perplexities 

 are introduced by the fact that the use of a wide pencil 

 of light is a desideratum, in order to gain sufficient illu- 

 mination when large magnification is to be secured. 



In the attempt to overcome these difficulties, the fore- 

 most physical philosophers of the time came to the aid 

 of the best opticians. Yery early in the century, Dr. 

 (afterwards Sir David) Brewster, the renowned Scotch 

 physicist, suggested that certain advantages might ac- 

 crue from the use of such gems as have high refractive 

 and low dispersive indices, in place of lenses made of 

 glass. Accordingly lenses were made of diamond, of 

 sapphire, and so on, and with some measure of success. 

 But in 1812 a much more important innovation was intro- 

 duced by Dr. William Hyde Wollaston, one of the great- 

 est and most versatile, and since the death of Cavendish 

 by far the most eccentric, of English natural philosophers. 



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