REFRJNGIBILirr of LIGHT. 55 



alkali of equal denfity. By fimilar trials, the phofphoric and 

 acetous acids were found to be confiderably more difperlive 

 than fpirit of wine. 



Although thefe experiments with compound object-glafles 

 of very large apertures, afford both the readied and moft accu- 

 rate method of inveftigating the optical properties of refracting 

 mediums, it would be both amufing and inftructive to repeat 

 them with compound prifms. I could have wifhed in parties 

 lar, had my prefent fituation been convenient for the purpofe, 

 to have taken the dimenfions of the fecondary fpectrum, under 

 given angles of incidence and refraction. For by comparing 

 thefe with the dimenfions of the primary fpectrum, accurately 

 afcertained by Sir Isaac Newton, the degree of fuperiority 

 of an objed-glafs compofed of crown and flint glafs, over a 

 fimple object-glafs, and of one in which there is a regular re- 

 fraction of all the rays, over both, might be afcertained. At 

 prefent, I can only ftate the circumftances of a comparifon I 

 made between two compound object-glafTes of equal apertures, 

 but very unequal lengths. One was compofed of crown-glafs, 

 fpirit of wine and an effential oil. The focal length is about 

 fourteen inches, and the aperture two inches. The other ob- 

 ject-glafs was of crown and flint-glafs ; its focal length thirty- 

 two inches, and its aperture two inches. I had it for a good 

 one of its kind, and upon examination found no particular de- 

 fed in its ftructure. 



The ihort telefcope has a manifeft advantage in the night, 

 efpecially in viewing fine objects, fuch as double ftars of infe- 

 rior magnitudes, where the uncorrected colour is lefs hurtful. 



But I was furprifed, on viewing an object in bright fun- 

 fhine, to find confiderably more of that miftinefs which arifes 

 from the unequal refrangibility of light, than appeared in the 

 long telefcope. I therefore diminifhed the aperture of the 

 ihort one to one inch and a half, and comparing them again> 

 there appeared no more of this mift in the one than in the 



©their* 



