1 a CmJIiiicHoh of Aplaiiatic Leiifes, 



fluid fit fov optica; ufes ; but nothing could be better adapted for this purpofc than metallic 



folutions. 



The firft tri.il was made with butter of antimony. On increafing the proportion of ma- 

 rine acid, the fringes of green and purple, which, being the intermediate rays, were irre- 

 gularly refracted by the metallic folution, grew narrower and narrower, till they entirely 

 difappearcd -, and if more was then added, they re-appeared in an inverted order. The fame 

 thing was tried with a folution of crude fal ammoniac and corrofive fublimate. With a cer- 

 tain proportion of thcfe two fubftances, the rays of all colours emerge from the compound 

 objecl-glafs equally rcfraded. If the proportion of the ammoniacal fait, and confcquently 

 of the marine acid it contains, be increafcd, the green rays which were the mean refrangible, 

 in the difperfive fluid as well as in crown-glafs, draw nearer to the violet, inaking a part of 

 the more refrangible half of the fpectrum, and confcquently emerge lefs rclracled than the 

 united red and violet rays, and arc converged to a focus at a greater dillance from the 

 obje£l:-glafs ; fo that the green fringe no«' appears within the focus, and the purple fringe 

 beyond it. But, on increafing the proportion of mercurial particles, tliefc fame green rays 

 ftiift their fituation to the lefs refrangible half of the fpedlrum, which appears from their now 

 emerging moft refratled, and being converged to a point nearer to the obied-glafs than the 

 united red and violet, whofe refrangibility does not appear to be affeded by thcfe admixtures 

 which occafionJuch remarkable fluftuations in the refrangibility of the green rays and other 

 intermediate orders. It may polTibly feem ftrange at fird; view, that the green rays fliould 

 emerge moft refraded from the compound objetfl-glafs, when their refrangibility in the 

 4ifperGve medium is diminiflied, and leaft refracted under the contrary circumftances. The 

 caufe of this is, that the principal refraction of the compound objciSt-glafs is performed by 

 the indifperfive convex lens, which is oppofite to the refraction produced by the difperfive 

 concave. 



Fig. 7. reprefents an objccl-glafs of this kind in Dr. Blair's pofrefiion, in which the 

 metallic particles are fo far diminilhed, and the particles of marine acid fo far increafed, as 

 to render the refraftion of the feveral orders of rays proportional in both mediums. 

 There arc two refractions in the confine of glafs and the fluid, but not the leaft colour 

 whatever. Hence it is inferred that, notwithftanding the confiderable difference of denfity, 

 and the refraction which, on account of the anterior furface being plain, and the pofterior 

 furface fpherical, having the focus of parallel rays for its centre, is performed only at the 

 two furfaces of the fluid, there is no unequal refrangibility of light. The rays of dif- 

 ferent colours are, as the Doflor obferves, bent from their rectilineal courfe with the 

 fame equality and regularity as in refra£tion. 



As cuftom has already appropriated the word Achromatic to that kind of refraClion in 

 which there is only n partial corrcftion of colour, the author propofes to diftinguifh this 

 entire removal of aberration by the term Aplanatic. 



Thus far 1 have endeavoured, partly by abridgement, partly by extra£ts, and partly 

 by taking greater liberties of narration and arrangement, to convey the fubftance of 

 Dr. Blair's important difcoverics to the public. It is now fome years fince the 

 attention of the world was lirft direCled to his improvements. I have therefore been 

 induced to make enquiry, anv^ng the artifts of tliis metropolis, into tlie ftate of the 



undertaking 



