REFRANGIBILITY of LIGHT. VW 
Cafes of refraction in which the violet rays are leaft refrangible, and 
the red rays moft refrangible; or in which all the rays are equally 
refrangible; or in which the red rays are refracted from the per- 
pendicular, and the violet rays towards the perpendicular, while 
the mean refrangible rays fuffer no refraction. 
Ir has been mentioned, that when prifms of crown-glafs and 
oil of turpentine refraét in oppofition, the tranfmitted light is 
colourlefs, when the proportion of the refracting angles of thefe 
prifms is as feven to fix. Hence, if oil of turpentine be in- 
cluded between two double convex lenfes, the radii of whofe 
comvexities are as fix to one, and the deep fides of thefe be 
placed inwards, fo as to be in contact with the fluid ; in the re- 
fraction through this compound lens, the aberration from 
the difference of refrangibility will be removed. I can prove 
the truth of what I write, by a compound object glafs of this 
kind, which I have had in my poffeflion above four years. It 
is twenty inches in focal length, and its performance as a tele- 
fcope, with one inch and a half of aperture, is not contempt- 
ible. Now, it has long ago been afcertained, that the mean 
refractive denfity of oil of turpentine is lefs than that of glafs ; 
and thence I affirm, that when light paffes from crown-glafs 
into oil of turpentine, a confiderable refra€tion of the whole 
pencil from the perpendicular takes place, and the violet rays 
are, in this cafe, the leaft refrangible, and the red rays the moft 
refrangible. . 
Tuis is manifeft from the faéts which have juft been ftated. 
In the objeét-glafs above mentioned, there are four refraétions, 
all of which are made in the fame direétion ; ‘namely, two re- 
frations at the two external furfaces of the lenfes, which are in 
contact with air, and two at the internal furfaces, which are in 
contact with oil of turpentine. 
Vor. IU. Gi In 
e 
