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XLVI. Note on Elementary Nomenclature in Geometrical 



Optics. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



THE notation of the paper, " Notes on Geometrical Optics," 

 by Professor Silvanus P. Thompson, in the Philo- 

 sophical Magazine for October, is in many respects simpler 

 than that used in current text-books. Sir W. Thomson, in 

 lecturing to the Natural Philosophy Class in this University, 

 uses a nomenclature which, so far as I know, has not yet 

 been published. I have his permission to give the following 

 summary of it, which is virtually a copy of a cyclostyled 

 paper that is put into the hands of each student when Sir W. 

 Thomson commences his lectures on Optics. 



Nomenclature. 



(1) The refractivity of a substance is the difference between 

 the index of refraction of the substance and unity. 



(2) The potency of a lens depends on two factors, refrac- 

 tivity and curvature. It is equal to the product of the refrac- 

 tivity into the algebraic sum of the curvatures of the lens. 

 The potency of a lens is called convergivity when it is for 

 convergence, and divergivity when it is for divergence. 



(3) The convergence or divergence of a pencil of light is the 

 reciprocal of the distance of the source, or of the image of 

 the source, from the centre of the lens. 



(4) Either convergence or divergence is altered by ad- 

 dition or subtraction of the potency. 



(5) Convergence of a pencil of light after passing through 

 lens = convergence of incident pencil + convergivity of lens. 

 Divergence of a pencil of light after passing through lens 

 = divergence of incident pencil — convergivity of lens, or 

 = divergence of incident pencil 4- divergivity of lens. 



Notation. 



(P) Refractivity =(ft — 1), where fi is the index of refrac- 

 tion of the substance. 



(2 X ) For a double convex lens, as in fig. 1, 



Convergivity = (^-1)(- + -,J- 



