COREESPONDENCE. 137 



does Coddington (both Senior Wranglers), the aberration of a con- 

 vergent lens is negative, the expression for which commences with the 

 negative sign ( — ). Thus : " The aberration of the ray of S = u' — v 

 (page 119) 



fx- \\r u J \r ti J \s V J \ s V J 2 ' 



The distasteful sentence is thus given in the Transactions of the 

 Eoyal Society, in a note, vol. ii., 1870 : — 



" It is convenient to define the aberration to be positive or nega- 

 tive, or the lens to be over- or under- corrected, by the simple fact 

 that a convex lens causes the excentrical rays to cross the axis at a 

 point nearer the centre of the lens than the centrical rays ; in which 

 case, and in all analogous cases, it may be said that the lens is under- 

 corrected and afflicted with a negative aberration." 



The language of mathematicians unfortunately somewhat differs on 

 this point from the rule of thumb employed in the workshop. In 

 mathematical language, if a line is measured to the right from a fixed 

 point be called positive, that measured to the left is negative. Accord- 

 ingly as the marginal rays of a convex lens fall nearer to it than 

 the central rays, the aberration is called negative. Any ordinary 

 convexo-j)lane, plano-convex or double convex, is said in mathema- 

 tical language to leave a negative aberration, and this lens is under- 

 corrected and vice versa. 



Now, Sir, as it is due to myself to give this explanation, after the 

 manner in which this statement has been treated, I may be allowed to 

 draw your attention to the construction of the positive and negative eye- 

 pieces, and especially to the principles upon which these names have 

 been bestowed by mathematicians upon these combinations. 



I am, Sir, your obedient servant, 



G. W. EOYSTON-PlGOTT. 



Db. Pigott and Sir David Brewster. 



To the Editor of the ' Monthly Microscopical Journal^ 



February 12, 1872. 



For the credit of the Microscopical Journal, and in justice to the 

 scientific character of the late Sii' David Brewster, Dr. Eoyston-Pigott, 

 upon having the matter pointed out, may probably offer some explana- 

 tion as to certain statements he is reported to have made in the 

 current number of your Journal, at page 87. 



Dr. Pigott, referring to photography, states that "Sir David 

 Brewster has laid down the principle that you cannot get a true 

 picture if you use a very large object-glass upon a small object, 

 on account of certain distortions, which, according to Sir David's 

 principle, are caused by the mixing up together of all the images 

 caused by different areas of the lenses, the result being a compound 

 image but not a likeness : " he adds that, " to show the portrait cor- 



L 2 



