I. 



PAPERS OF THE PHTSICAL CLASS. 



I. Experiments and Observations on the Unequal Re- 



FRANGIBIL1TY of LlGHT. By ROBERT BLAIR, M. D. 



[Read Jan. 3. and April 4. 1791.] 



BY the difcovery of the different refrangibility of light, Sir 

 Isaac Newton laid open the true caufe of the principal 

 imperfection of refracting telefcopes ; and having inferred from 

 the experiments which he made, that the refraction of the dif- 

 ferent rays compofing the prifmatic fpectrum, was always in a 

 given ratio to the refraclion of the mean refrangible ray, this 

 great philofopher was led to conclude, that the imperfection 

 which he had difcovered in dioptrical instruments was without 

 remedy. 



If Sir Isaac Newton had been queftioned concerning the 

 poflibility of refracting light, without any divergency of the 

 heterogeneal rays, his reply without doubt would have been, 

 that all his experiments, whether by {ingle refractions or by 

 oppofite refractions, tended to eflablifh the contrary conclulion. 

 But that he would have aflerted nothing beyond this, may fafe- 

 ly be inferred from his own memorable words : " Although 

 " the arguing from experiments and obfervations by induction 

 u be no demonstration of general conclufions, yet it is the belt 



A ' 2. " way 



