Cnap. 9.] from the Ref Tangibility of Light. 267 



ing, in great meafure, their theories. By making a 

 compound lens of three different fubftances of differ- 

 ent refrangible powers, the rays of light, which were 

 difperfed too much by one convex lens, are brought 

 nearer to an union with each other, and the telefcopes 

 made with an object glafs of this kind are now com- 

 monly ufed, and well known by the name of achro- 

 matic telefcopes -, the word achromatic being ufed by 

 that pedantry which infects mod of our philofophers, 

 who love to give a Greek word, unintelligible to the 

 greater part of their readers, inftead of the equally fig- 

 nificant term in our own language, colourlefs. 



1 The object-glarTes of Mr. Dollond's telefcopes arc 

 compofed of three diftinct lenfes, two convex and one 

 concave j of which the concave one is placed in the 

 middle, as is reprefented in Fig. 51. where a and c 

 fhow the two convex lenfes, and b b the concave one, 

 which is by the Britifh artifts placed in the middle. 

 The two convex ones are made of London crown 

 glafs, and the middle one of white flint glafs 3 and they 

 are all ground to fpheres of different radii, according 

 to the refractive powers of the different kinds of glafs, 

 and the intended focal diftance of the object glafs of 

 the telefcope. According to Bofcovich, the focal dif- 

 tance of the parallel rays for the concave lens is one- 

 half, and for the convex glafs one-third of the com- 

 bined focus. When put together, they refract the rays 

 in the following manner. Let ab, ab (Fig. 52.) be 

 Itwo red rays of the fun's light tailing parallel on the 

 firft convex lens c. Suppofmg there was no other lens 

 prefent but that one, they would then be converged 

 into the lines be y be, and at lail meet in the focus q. 

 Let the lines gh, gb, reprefent tv. o violet rays falling 

 on the furtttC.e of the lens. Theie are aifo refracted, 

 and will meet in a focus j but as they have a greater 



