REFRANGIBlLlTr of LtGHf. 17 



Cafes of refraclion in which the violet rays are leajl refrangible, and 

 the red rays mojl 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 refraclion. 



It has been mentioned, that when prifms of crown-glafs and 

 oil of turpentine refract 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- 

 eluded between two double convex lenfes, the radii of whofe '' 

 convexities 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 pofTeffion 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 pafTes from crown-glafs 

 into oil of turpentine, a confiderable refraction 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. 



This is manifeft from the facts which have juft been dated* 

 In the object-glafs above mentioned, there are four refractions, 

 all of which are made in the fame direction ; namely, two re- 

 fractions 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. 



Vol. III. C In 



