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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 pafles obliquely out of crown-glafs 
into oil of turpentine, it is refracted from the perpendicular, 
and the red rays are moft refraéted, 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 
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, inftead of being removed by thet moult 
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 
conftrution 
