j6 On the UNEQUAL, &c. 



which it is for the red and violet rays. Thefe three orders of 

 rays will therefore fufFer an equal refra<fUon, being each of 

 them attracted towards the glafs with the force two ; and when 

 this is the cafe, it appears from the obfervations, that the inde- 

 finite variety of rays of intermediate colours and (hades of co- 

 lours, which altogether compofe folar light, will alfo be regu- 

 larly bent from their rectilinear courfe, conftituting what has 

 been termed aplanatic refraction. 



These cafes of attraction might be farther illuftrated by 

 means of diagrams. But after the explanation already given 

 of the fecond, third, fourth, fifth, fixth, feventh and eighth fi- 

 gures, this would be unnecefTary. And it need fcarcely here be 

 obferved, that the above rough ftatements in round numbers, 

 are intended to give a clear idea of the nature of the various 

 cafes of unequal refrangibility, and not to afcertain its quantity 

 in any particular cafe. A full inveftigation of the fubject, and 

 an account of fome digremons lefs immediately connected 

 with the principal object which occurred in the courfe of the 

 enquiry, could not be brought within the compafs of the pre- 

 fent communication. 



II, 



