REFRJNGIBILirr of LIGHT, 31 



On this account I attempted to introduce a third medium, by 

 filling this vacancy with a fluid of the leaft difperfive kind, and 

 of lefs mean refractive denfity than the difperfive fluid. For 

 this purpofe I employed fometimes rectified fpirit of wine, and 

 fometimes vitriolic aether j and by giving to the lenfes the pro- 

 per degree of curvature, in which great variety may be intro- 

 duced, I fucceeded in forming object-glafTes, in which both 

 aberrations are removed, and hardly any more light loft than 

 in a Ample objecl-glafs. 



Having gained this point, I now determined to try how 

 far the aperture of the object-glafs might be increafed, without 

 increafing its focal length, expecting, at leaft, to equal reflectors 

 in this refpect. But the firft trials to execute object- glafles on 

 this principle, though they left no reafon to complain of want 

 of fuccefs, when compared with fuch inftruments as are now in 

 ufe, furprifed me with new phenomena, and new obftacles to 

 the perfection of the theory of telefcopes, more unaccountable 

 and perplexing than any I had before encountered. Thefe I 

 fhall now proceed to give an account of. 



Of the imperfecl correction of prifmatic colour which is obtained by a 

 combination of mediums of different difperfive powers. 



I took a compound object-glafs of the conftru&ion laft 

 mentioned, compofed of three lenfes, two of them plano-con- 

 vex and the other a menifcus. The radius of convexity of one 

 of the plano-convex lenfes is about four inches, and the con- 

 vex fide is turned towards the object. The radii to which both 

 fides of the menifcus are ground, are about five inches, one 

 fide being convex and the other fide concave. The concave fide 

 is made to refpect the plane fide of the above mentioned plano- 

 convex, and the vacancy between them is filled with vitriolic 

 aether. The third piano convex lens is ground to a radius of fix 

 inches. Its convex fide is turned towards the convex fide of 



the 



