354 P. C on the Colours that enter into the [Nov. 



were in the proportion which forms white light, all these 

 objects would be white, without the least appearance of 

 colour. This is precisely what we should have in the field 

 of view with a perfect telescope. 



But if, instead of being of the same dimensions, we sup- 

 pose the pictures, preserving their correspondence in every 

 other respect, to be drawn upon different scales, the violet 

 picture, for instance, a little larger than the green, and the 

 green a little larger than the red, it is obvious that when 

 they are placed upon each other, the want of correspondence 

 will be found throughout the whole ; and though the defect 

 will gradually increase from the centre as we proceed to 

 the circumference, there will not be a single object the 

 three images of which will perfectly coincide. It will be 

 no remedy for this defect in the telescope, therefore, to 

 absorb the violet which extends at the edges beyond the 

 green; and the blue, formed by the green and violet, 

 which is seen beyond the red, where all the colours meet 

 and form white ; unless we extend the operation to every 

 part of every object in the field of view, and thus render 

 the whole monochromatic : any thing short of this might 

 still leave the head of one animal in red light upon the 

 shoulders of another in violet light, and, by other trans- 

 fers, upon the same principle, produce such confusion as 

 would, at least, preclude the possibility of a distinct percep- 

 tion of minute objects : it would be like cutting the edges 

 of the pictures we have been speaking of, so as to reduce 

 them to the same dimensions, leaving minute objects, re- 

 presented upon them, mixed together in endless confusion, 

 and the different parts of larger objects in the same disjointed 

 state, though they would in some degree preserve their pro- 

 per colours by a system of mutual compensations. 



This defect is lessened, but not cured, in the achromatic 

 telescope. Sir D. Brewster informs us that " all achromatic 

 telescopes whatever, when made of crown and flint glass, 

 exhibit the secondary colours, viz. the wine-coloured s.nd the 

 green fringes." (p. 369.) The wine-coloured fringes are 

 formed of red and violet, of which green is the complemen- 

 tary colour ; the former are seen upon one edge of white 

 objects, and the latter upon the opposite edge, from the 

 want of exact coincidence between the differently coloured 

 images ; and they cannot be entirely removed but by either 



