18 On the UNEQUAL 



In the refractions which take place in the confine of glafs 

 and air, it has been put beyond all doubt, by Sir Isaac New- 

 ton's experiments, that the red rays are leaft refracted, and the 

 violet rays moft refracted-; and it is equally clear, from what 

 has juft been mentioned to be the refult of trials with prifms, 

 and from the correction of colour in the above mentioned ob- 

 ject-glafs, that when light pafTes obliquely out of crown-glafs 

 into oil of turpentine, it is refracted from the perpendicular, 

 and the red rays are moft refracted, and the violet rays leaft 

 refracted. If this were otherwife, the heterogeneal rays, which 

 are made to diverge in two refractions, which take place in the 

 e confine of glafs and air, could never have this divergency re- 

 moved by the refractions made in the confine of glafs and the 

 fluid. It is manifeft, that if, in thefe laft mentioned refrac- 

 tions, the feparation of the heterogeneal rays were in the fame 

 order as in the refraction from air into glafs, the colour and di- 

 vergency of the rays, inflead of being removed by them, would 

 be increafed. 



I shall not enter upon the application of this fact to the 

 beft received theories of refraction ; but it may be worth while- 

 to remark the great importance of minute accuracy in obferving 

 the refults of experiments. Dr Hook attempted to make ob- 

 ject-glaffes of telefcopes, by interpofing a fluid between a plano- 

 convex lens, and a piece of glafs, both fides of which were 

 plane and parallel. The convex fide of the lens was turned 

 inwards ; and the author feems to have had no other view in 

 this fcheme, but to obviate the difficulty which was found in 

 giving a good figure to lenfes ground to very long radii. The 

 refraction being thus reduced to that which takes place in the 

 confine of glafs and the fluid employed, may be diminifhed in 

 any proportion, and confequently the focal length of the object- 

 glafs lengthened at pleafure. One of the fluids which he ap- 

 pears to have made ufe of, was oil of turpentine. The diffe- 

 rence between the phenomena attending an object-glafs of this 



conltruction 



