The Achromatic Telescope. 261 



caused as mucli surprise at the time, as we may now feel at its 

 having been kept secret. A strange obscurity has veiled the 

 name of a man whose inventive genius and mathematical 

 acquirements must have been of no common order. His 

 success was so completely unknown that the subject was taken 

 up as an unexplored one, fourteen years afterwards, by the 

 celebrated Euler, in a manner which excited the attention of 

 John Dollond, a London optician, whose parents had quitted 

 Normandy in consequence of the revocation of the Edict of 

 Nantes. The mathematical abilities of this remarkable man 

 enabled him to investigate the subject thoroughly, and the 

 result was the re-discovery of the important fact that the 

 amount of refraction being equal, that of dispersion varies 

 according to the nature of the refracting medium. The appli- 

 cation of this principle at once produced the achromatic 

 telescope, which was secured by patent, notwithstanding some 

 opposition on the ground of Hall's previous success, Lord 

 Mansfield remarking that " it was not the person that locked 

 up his invention in his scrutoire that ought to profit by a 

 patent for such an invention, but he who brought it forth for 

 the benefit of the public*" 



We have now only to premise that the property of a concave 

 lens is the exact reverse of a convex one, producing a diverging 

 instead of a converging pencil of rays, and we shall readily 

 comprehend this truly ingenious construction. 



There are two kinds of colourless glass, differing consi- 

 derably in their optical qualities. These are Plate glass, the 

 material of mirrors (which has replaced the greenish crown 

 or window glass of the older opticians), and Elint glass, that 

 used for drinking vessels, which contains a proportion of lead, 

 whence arises its peculiar action upon light. Of these, when 

 the refractions are equal, the flint has a considerably greater 

 dispersive power. If, then, we were to combine a convex lens 

 of plate with a concave of flint of equal focal length, though 

 the rays would never come to a focus, passing through parallel, 

 from equal refraction in opposite directions, there would still 

 be a balance of dispersion on the side of the flint, and colour 

 would be produced, which the plate would be unable to neu- 

 tralize. Now, if we give more refractive power to the convex 

 lens by shortening its focal length, we also increase the absolute 

 amount of its dispersion ; and by repeated trials we shall find 

 it possible to make this dispersion the exact counterbalance of 

 that of the flint lens, so that all colour shall disappear, while 

 the shortened focus of the convex plate, preponderating over 

 the unchanged divergent power of the concave flint, will cause 

 the rays, instead of passing through both lenses parallel, to 

 converge to a focus. In that point an image of external objects 



