CORRESPONDENCE . 4 5 



an inch, which is traversed by the moving knife from heel to point, 

 and with so gentle a sway that perfect sections are readily made. 



With this microtome I have produced sections as large as the 

 cylinder would allow, especially of nerve matter ; but it answers just 

 as well for other tissues if properly prepared for cutting, an indis- 

 pensable condition. 



Christopher Johnston, M.D. 



Chromatic and Spherical Aberration distinct. 



To the Editor of the ' Monthly JiJicroscopical Journal.' 



16, FiTZROY Square, W., November 20, 1875. 



Sir, — I fail to perceive what benefit can result to science from an 

 endeavour to confound two things known by distinct characteristics, 

 and designated by different terms : it tends (as it ajjpears to me) to 

 meddle with, and muddle, rather than to elucidate scientific nomen- 

 clature. I allude to a paper in your last issue by Dr. Eoyston- 

 Pigott, " On the Identical Characters of Chromatic and Spherical 

 Aberration." Aberration is I take it — as its derivation imj^lies — " a 

 straying away from." Spherical aberration is a straying away from 

 the focus of rays not very near the axis of the pencil reflected, or of 

 homoyeneous rays refracted, at a spherical surface ; and chromatic 

 aberration is the straying away of all other rays from the focus of 

 rays of any given colour, in consequence of their unequal refrangibility. 



Dr. Pigott states that the spherical aberration of a homogeneous 

 pencil "is for convenience called chromatic aberration:" to this 

 statement I must entirely demur. In the succeeding paragraph he 

 is startled by the statement that all chromatic aberration does not 

 involve si:)herical aberration. In a lens with proper elliptic surfaces 

 (if it could be constructed) there would be chromatic but no sjiherical 

 aberration ; and Sir J. Herschel (I think) has given a formula by 

 which the spherical aberration of a lens may be corrected by a 

 meniscus of the same glass, but the chromatic aberration would 

 thereby be much increased. 



It is perfectly true that in all lenses chromatic and spherical 

 aberrations must be coexistent, because the only practicable sur- 

 faces are spherical ; but coexistence and identity are not synonymous 

 terms. 



I remain, Sir, yours faithfully, 



Charles Brooke. 



Chromatic and Spherical Aberration. 



To the Editor of the ^Monthly Microscopical Journal.' 



Sir, — I have made so many attacks against Dr. Pigott's strange 

 optical announcements — for such I have considered them — that I had 

 determined to allow his paper — the strangest of them all — concerning 

 chromatic and spherical aberration to rest without comment, as its 

 erroneous teaching must be evident to everyone whose instruction is 



