REFRANGIBILITT of LIGHT. to. 



construction and a fimple lens, if they had been attentively ob- 

 ferved, would have led Dr Hook to the truth ; and a man of 

 his zeal and invention would not have failed to apply the dif- 

 covery to the improvement of optics, not to mention the tri- 

 umph it would have afforded the opponent and rival of New- 

 ton, to have aflerted, and had it in his power to make good 

 his affertion, that in fome cafes the violet rays are the lead re- 

 frangible, and the red rays the moft refrangible. 



Even Mr Dollond could not conceive that the prifmatic 

 colour could be corrected by refractions which are all made 

 the fame way ; and ftill lefs would he have admitted that An- 

 gle refractions may take place without divergency or colour *. 

 As this continues to be the opinion of the bed informed opti- 

 cians of the prefent day, it will be necefTary to enter into a more 

 explicit investigation of the fubject. 



Fig. 2. Let ABC reprefent a glafs prifm, and BCD a 

 prifm of water in contact with it ; and let the angles of thefe 

 prifms be fo proportioned to each other, that a ray of light S I, 

 which enters the glafs prifm perpendicularly, fhali, after being 

 refracted from the perpendicular at the point G, in paffing out 

 of the glafs into the water, emerge at K, perpendicular to the 

 fide C D of the water prifm, which is fuppofed to be confined by 

 parallel plates of glafs. As the ray both enters and emerges 

 from the refracting mediums perpendicularly, it will fuffer no 

 refraction, excepting when it pafles from the glafs into the wa- 

 ter, where its incidence is oblique. Here it will be refracted 

 from the perpendicular, and will emerge coloured, the violet 

 rays being moft refracted, and the red rays leaft refracted. 



Let the water be now impregnated with antimony or mer- 

 cury, to increafe its difperfive power. As this will alfo increafe 

 its mean refractive denfity, and occafion a diminution of the 



C 2 refraction 



* Philofophical Tranfa&ions of London, vol. 1. page 740. 



