lo Vjikus Jlifrrat'ions in eompiiiiid Lenfcs. 



wliich amounts pvccilcly to the f.ime thing, iliminilli the difference of denfity of tlic two 

 mediums at pleafure. 



It will appear from wliat has been explained, that the aberration from the figure cannot 

 be correclcd by interpofing a difperfive fluid between two convex lenfcs of a greater re- 

 fratlive denfity than the iiiterpofed fluid. For all the rcfradions, being made tlic fame 

 way, tend to converge the external rays to points nearer the lens than its geometrical focus. 

 Hence, when rare fluids are made ufe of to remove the aberration from the difference 

 of refrangibility, fome f.irther contrivance becomes neceffary to corrcdl the fplerical 

 aberration. 



The mod obvious v.-av, and whicli on trial was found fuccefsful, is to include the rare 

 difperfive fluid between two glaffes, ground concave on one fide and tonvex on the other, 

 and thus form fuch a concave as fliall be required. An Achromatic Objefl-glafs may be 

 formed by combining this with a convex. The objection to this conftruftion is, that one of 

 the advantages arifing from the ufe of fluids is given up, namely, the prevention of that 

 lofs of light, by refleclion, which is a confequence of the fluid being in immediate contact 

 with the glafs, whereas in the prcfent cafe the fpace between the convex and concave is 

 occupied by air. 



On this account. Dr. Blair attempted to introduce a third medium, by filling this vacancy 

 with a fluid of the leaft difperfive kind, and of lefs mean refra£live denfity than the dif- 

 perfive fluid. For this purpofe he employed fometimes reftified fpirit of wine, and fome- 

 times vitriolic ether -, and by giving to the lenfes the proper degree of curvature, in which 

 great variety may be introduced, lie fucceeded in forming obje£l-glaffes in which both 

 aberrations are removed, and hardly any more light loft than in a fimple objeft-glafs. 



Having gained this point, he now determined to try how far the aperture of the obje£t- 

 glafs might be increafed, without increafing its focal length, expefting, at leaft, to equal 

 refleilors in this refpe^El. But the firft trials to execute obje^-glaffes on this principle, 

 though they left no reafon to complain of want of fuccefs when compared with fuch in- 

 ftruments as are now in ufe, exhibited new phenomena and new obftacies to the perfeilion 

 of the theory of Telefcopes more unaccountable and perplexing than any he had before en- 

 countered. The hiftory of thefc interefting facts, and the regular progrefs of difcovery 

 by which they were remedied, conftitute a large part of the paper, which well deferves 

 to be confulted by thofe who wifti effeittualiy to profecute this fubject. Brevity, however, 

 demands a lefs hiftorical narration in this place. 



Thefc new difRculties arofc frojri the effects of contrary powers of difperfion, which 

 though equal upon the extreme rays, were not found to be the fame upon the rays that oc- 

 cupy intermediate fpaccs in the coloured fpeclrum. From what has been already ftated 

 on this fubjedl, the attentive reader may underftand, that fuch intermediate rays from a 

 compound lens of the kind here defcribed, would not be aflembled at the common focus. 

 The particular nature of this difperfion is much more accurately feeii by applying a deep 

 eyc-glafs to the focal image, tlun by experiments with piifms. When the image of a lucid 

 point is formed in the focus of a fimple lens, the violet or moft refrangible rays are con- 

 verged to a focus nearcft to the lens, and the deep red rays are converged to a focus at the 

 grtateft diftance from it. The confequence of this is, that if the image be examined by an 

 eye-glafs nearer to die lens than is required for diftinil vifion, it will be furrounded with a 



red 



