THE PRESIDENTS ADDRESS. 



147 



light into its component colours. Red focusses on the axis AA' 

 at R, and violet at V, some of the other colours being omitted 

 for clearness of diagrammatic representation. This is the per- 

 formance of the simple uncorrected lens. A great improvement 

 is at once effected by combining a convex crown lens with a 

 concave flint ; the four radii and the different dispersive properties 

 of the two glasses enable the computer to, — 



(1) Bring the central and marginal rays of one colour to focus 

 at the same point — in other words, to cure spherical aberration 

 for one colour, any wave-length that is chosen being spoken of 

 as the " preferred colour " ; 



(2) To correct coma for the same colour — in other words, to 

 fulfil the demands of the sine-law for this preferred colour ; and 



(3) To cause the rays of other colours to concentrate very 



FmX 



VISUAL ACHROMATISM 



ACH RCMATISEDfor QREEN 

 A SLIGHTLY "UMDERCCRRECTED* 

 ORDINARY^CHROMATIC 



nearly upon the same point on the axis as those of the pre- 

 ferred colour. 



The lens is now said to be achromatic in the ordinary sense of the 

 w 7 ord. If, however, the computer has over-corrected in the matter 

 of colour, we shall have a state of things shown in Fig. 8, where 

 red has become the shortest focus and violet the longest ; so we 

 call this total over-correction for chromatic aberration. If we 

 examine Fig. 9 and those diagrams which immediately follow, 

 we shall quickly notice that the focus of the preferred colour is 

 always the shortest ; so the spectrum may justly be said to be 

 folded over at that point, for should the selected colour be yellow- 

 green — which is, in fact, that usually taken for visual objectives — 

 yellow-green will be seen to have the shortest focus ; orange and 

 blue are folded together, and further on red and violet. Achro- 

 matisation for green — what has been termed a slightly under- 

 corrected ordinary achromatic — we see depicted in Fig. 10. Here 

 the preferred colour has again the shortest focus, a blending of 

 yellow and blue coming next, whilst red and violet are seen still 



