ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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cylindricity of tlie lens, the defect which produces astigmatism. What 
Mr. Mallock applies this term to is spherical aberration pure and simple. 
I pointed out in 1889, in my article in the ‘ Philosophical Magazine,’ 
how the focal power of a lens is the product 
of a number depending only on the properties 
of the material into the sum of the two 
curvatures of its faces, or, in the case of thick 
lenses of a more complex quantity depending 
on the thickness of the lens as well as its 
curvatures. And in order to facilitate calcula- 
tions I devised a special form of spherometer 
— the dioptrie spherometer — (fig. 19) which 
measures the curvatures of the surfaces 
directly in dioptries. This was also con- 
structed for me by Messrs. Nalder Bros. To 
apply this to any simple lens one merely takes 
Fig. 20. 
the two readings, adds them, and multiplies by a constant * which 
depends only on the material. For crown glass the constant is 0 • 54 ; 
and for flint glass it varies from 0‘ 64 to 0-78 according to density. 
* This is equal /u. — 1, where n is the refractive index ; or (1 — h) -h h, where h 
is the velocity-constant (relatively to air). 
K 2 
