REFRANGIBILITY of LIGHT. 55 
alkali of equal denfity. . By fimilar trials, the phofphoric and 
acetous acids were found to be confiderably more difperfive 
than fpirit of wine. 
- AttuoucesH thefe experiments with compound Bh eee glafles 
of very large apertures, afford both the readieft and moft accu- 
rate method of inveftigating the optical properties of refracting 
mediums, it would be both amufing and inftrutive to repeat 
them with compound prifms. I could have wifhed in particu- 
lar, had my prefent fituation been convenient for the purpofe, 
to have taken the dimenfions of the fecondary fpe@trum, under 
given angles of incidence and refraction. For by comparing 
thefe with the dimenfions of the primary fpetrum, accurately 
afcertained by Sir Isaac Newron, the degree of fuperiority 
of an object-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, 1 can only ftate the circumftances of a comparifon I 
made between two compound object-glaffes 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- 
fect in its ftructure. ‘ 
» Tue fhort telefcope has a manifeft advantage in the night, 
efpecially in viewing fine objects, fuch as double ftars of infe- 
rior magnitudes, where the uncorre¢ted colour is lefs hurtful. 
Bur 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 
fhort one to one inch and a half, and comparing them again, 
there appeaed no more of this mift in the one than in the 
other.. 
