THE JOURNAL 



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ABBE'S TEST OF APLANATISM, AND A SIMPLE 

 APERTOMETER DERIVED THEREFROM. 



By Frederic J. Cheshire, F.R.M.S. 



(Bead Ju?ie 19th, 1903.) 



The Abbe-Helmholtz sine-law expresses, as is well known, the 

 necessary and sufficient condition for the production, by the 

 different zones of a wide-angle optical system, of equal -sized 

 images of an indefinitely small object on the axis of the system, 

 and in a plane at right angles to that axis. 



Fig. 1. — Diagram of Aplanatic System. 



Let P and Q be a pair of conjugate and aplanatic foci, on the 

 axis of a wide-angle optical system, then the sine-law states that 

 the sine of the angle a, which any ray makes with the axis 

 in passing from the point P, must bear a constant ratio to the 

 sine of the angle /?, which the corresponding conjugate ray 

 makes with the axis, when passing through the point Q. If 

 the points p and Q be immersed in media with refractive indices 



Journ. Q. M. C, Series II. — No. 54. 1 



