202 correspondence. 



Chromatic and Spherical Aberration. 



To the Editor of the ' JS'lonthly Microscopical Journal.^ 



Chislehurst, March 15, 1876. 



Sir, — May I, as a very old member of the Microscopical Society, 

 be permitted a few remarks on the two papers by Dr. Eoyston-Pigott 

 which appeared in the ' M. M. J.,' No. Ixxxiii., p. 232, and No. Ixxxvii., 

 p. 128, which in my opinion are highly pernicious, as being calculated 

 to produce in the minds of the uninformed a hopeless confusion of ideas 

 between two things utterly distinct, and which the author himself 

 incidentally shows to be distinct in the course of his illustrations. 



I would premise, that in mathematical attainments I do not 

 presume for one moment to compare myself with this gentleman ; but 

 as a veteran worker with the microscope, who believes that an inti- 

 mate acquaintance with the theoretical and practical construction of 

 all the optical parts of the instrument is a necessity to an accurate 

 observer, I have for over five-and-thirty years studied the science of 

 optics with a special view to its practical aj)j)lication ; and I trust 

 that I may, without offence, express my strong dissent from the con- 

 clusions at which Dr. Eoyston-Pigott appears to have arrived, and to 

 express my conviction that, from some unexplained cause, he api^lies 

 another meaning to the term " chromatic aberration " than that gene- 

 rally accepted. 



At p. 131, No. Ixxxvii., of the ' M. M. J.,' some well-known experi- 

 ments illustrative of chromatic and spherical aberrations are described ; 

 but oddly enough, instead of pointing out that the differing positions 

 of the foci of the red and violet images of the sun, as formed by the 

 marginal rays only, or by the central rays only, illustrate the 

 chromatic aberration, and nothing else, the author dilates upon the fact, 

 that the different positions of the foci for the red image of the sun 

 when formed by the marginal or central part of the lens respectively, 

 and the same with the violet image, show, that both the red and 

 violet rays are subject to spherical aberration ; and the foot-note in 

 p. 132 of the same number is to the same effect. 



But this is a fact about which, so far as I know, there has never 

 been any dispute. Of course the rays of light of all colours are 

 subject to spherical aberration when transmitted through lenses with 

 spherical surfaces ; but that has nothing whatever to do. with chromatic 

 aberration, which arises solely from the various degrees of refrangi- 

 bility of the differently coloured rays. In point of fact, cJiromatic 

 aberration is due to the compound character of the light employed, 

 and has no existence with homogeneous light ; while spherical aberra- 

 tion is due only to the form of the lens employed, whether the light is 

 simple or compound ; and these facts are abundantly demonstrated by 

 the experiments detailed by Dr. Eoyston-Pigott himself. 



" Chromatic aberration " requires for its correction the use of at 

 least two refracting media (besides the air) of varying disj)ersive 

 powers, and cannot exist with monochromatic light. " Spherical 

 aberration " can be corrected or balanced for monochromatic light 



