ANATOMY AND PHYSIOLOGY 



crease of reflecting surfaces, Dr. Brews ter suggested 

 filling the interspace between the two lenses with a 

 cement having the same index of refraction as the lenses 

 themselves an improvement of manifest advantage. 

 An improvement yet more important was made by 

 Dr. Wollaston himself in the introduction of the dia- 

 phragm 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 peri- 

 scopic microscope. Dr. Brewster suggested that in 

 such a lens the same object might be attained with 

 greater ease by grinding an equatorial groove about a 

 thick or globular lens and filling the groove with an 

 opaque cement. This arrangement 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 physi- 

 cists of the time, also gave attention to the problem of 

 improving the microscope, and in 1821 he introduced 

 what was called an aplanatic combination of lenses, in 

 which, as the name implies, the spherical aberration 

 was largely done away with. It was thought that the 

 use of this Herschel aplanatic combination as an eye- 

 piece, combined with the Wollaston doublet for the 

 objective, came as near perfection as the compound 

 microscope was likely soon to come. But in reality 

 the instrument thus constructed, though doubtless 

 superior to any predecessor, was so defective that for 

 ticul purposes the simple microscope, such as the 

 doublet or the Coddington, was preferable to the more 

 complicated one. 



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