2 Mr. Barlow on the Refracting Telescope. 



Dollond. When it is made a parallel glass for the mean 

 rays, it agrees with the construction proposed by Mr. Rogers ; 

 and in all its intermediate states, it is the form which I have 

 proposed for the naturally high dispersive power of the sul- 

 phuret of carbon. 



It is certainly a little remarkable that the original idea of the 

 triple object-glass had never led to this view of the subject ; for 

 it is obvious that if, instead of comparing the combined focus 

 of the two crown lenses with that of the flint, we compare the 

 combined focus of the two back lenses with that of the front, 

 or the two front with that of the back lens, we arrive at a new 

 artificial dispersive ratio of the kind above alluded to, but the 

 author of this ingenious arrangement, having had in view only 

 the reduction of the spherical aberration, seems to have lost 

 sight of the other advantages he might have derived from the 

 same principle. 



In my endeavours to supply the place of flint glass by the 

 sulphuret of carbon, I was in some measure forced upon that 

 step which Mr. Dollond had failed to take, by finding it neces- 

 sary to open the lenses ; and this construction immediately re- 

 called to Mr. Rogers an idea which he had formed some time 

 before of that principle, which he has since published. But 

 neither this gentleman nor myself, any more than the inventor 

 of the triple object-glass, seems to have had any idea that we 

 had each penetrated by different roads upon the borders only 

 of an extensive field of uncultivated practical optics, which, if 

 duly explored by well directed experimental researches, cannot 

 fail of producing many valuable results. 



While we confine ourselves to crown and flint glass, the 

 range is certainly rather limited: if we employ the high re- 

 fractive glass of Mr. Faraday's manufactory, it is considerably 

 increased ; but with the sulphuret of carbon it can be so far 

 extended as to make the entire correction for colour, in an 

 object-glass of any diameter, in the eye-piece of the telescope 

 only. Mr. Coddington has, indeed, remarked, in his highly va- 

 luable * Treatise on Optics,' that my correcting lens must be 

 considered as a part of the eye-piece of the telescope ; but this 

 has reference only to its theory : what I allude to above is to 

 be understood also in a practical sense ; as it is possible to pro- 



