2, On the UNEQUAL 
and the quantity of light tranfmitted by equal apertures of the 
telefcopes much greater. 
Sucu are the advantages which the theory prefents. In re- 
ducing this theory to practice, difficulties muft be expected in 
the firft attempts. Many of thefe it was neceflary to furmount 
before the experiments could be completed. For the delicacy of 
_ the obfervations is fuch, as to require a confiderable degree of 
perfection in the execution of the objeét-glaffes, in order to ad- 
mit of the phenomena being rendered more apparent by means 
of high magnifying powers. Great pains feem to have been 
taken by mathematicians to little purpofe in calculating the ra- 
dii of the fpheres requifite for achromatic telefcopes, from 
their not confidering that the objet-glafs itfelf is a much nicer 
-teft of the optical properties of refracting mediums than the 
grofs experiments made by prifms, and that the refults of their 
demonftrations cannot exceed the accuracy of the data, however 
much they may fall fhort of it. 
I staxz conclude this paper, which has now greatly exceed- 
ed its intended bounds, by enumerating the feveral cafes of 
unequal refrangibility of light, that their varieties may at once 
be clearly apprehended. 
In the refraction which takes place in the confine of every 
known medium and a vacuum, rays of different colours are 
unequally refrangible, and the red-making rays are leaft re- 
frangible, and the violet-making rays are moftrefrangible. 
Tuts difference of refrangibility of the red and violet rays 
4s not the fame in all mediums. Thofe mediums in which the 
difference is greateft, and which, by confequence, feparate or 
difperfe the rays of different colours moft, have been diftin- 
guifhed by the term di/perfive, and thofe mediums which fepa- 
rate the rays leaft have been called indi/perfive. Difperfive me- 
diums differ from indifperfive, and {till more from each other, 
an another very eflential circumftance. 
Ir 
