THE STORY OF NINETEENTH-CENTURY SCIENCE 



This was the suggestion to use two plano-convex lenses, 

 placed at a prescribed distance apart, in lieu of the sin- 

 gle double convex lens generally used. This combina- 

 tion largely overcame the spherical aberration, and it 

 gained immediate fame as the " Wollaston doublet." 



To obviate loss of light 

 in such a doublet from in- 

 crease of reflecting surfaces, 

 Dr. Brewster suggested fill- 

 ing the interspace between 

 the two lenses with a ce- 

 ment having the same index 

 of refraction as the lenses 

 themselves an improve- 

 ment ot manifest advan- 

 tage. An improvement yet 

 more important was made 

 by Dr. Wollaston himself, 

 in the introduction of the 

 WILLIAM HYDE WOLLASTON diaphragm to limit the field 



of vision between the lenses, 



instead of in front of the anterior lens. A pair of lenses 

 thus equipped, Dr. Wollaston called the periscopic micro- 

 scope. Dr. Brewster suggested that in such a lens the 

 same object might be attained with greater ease by grind- 

 ing an equatorial groove about a thick or globular lens 

 and filling the groove with an opaque cement. This ar- 

 rangement found much favor, and came subsequently to 

 be known as a Coddington lens, though Mr. Coddington 

 laid no claim to being its inventor. 



Sir John Herschel, another of the very great physicists 

 of the time, also gave attention to the problem of im- 

 proving the microscope, and in 1821 he introduced what 



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