REFRANGIBILITY of LIGHT. 13 
Ir may be proper here to remark, that as it is the ratio of 
refraction of the mean refrangible ray which is wanted, the 
centre of the reflected image of the fun ought to be made to 
coincide with the centre of the coloured fpectrum, as repre- 
fented in the figure; and if, inftead of this, the coincidence, 
" 4s formed with the moft or leaft refrangible ray, or any of the 
intermediate rays, it will be the ratio of refraction of thefe 
rays, and not of the mean refrangible ray, which will be found 
from the obfervation. Hence this method might be practifed 
for determining the difperfive power, as well as thee mean re- 
fraCtive denfity of any tranfparent fubftance, whether folid or 
fluid; but I have preferred a combination of prifms or lenfes, 
becaufe it is the relative ratios, more than the abfolute ratios, 
which are moft immediately wanted. 
Experiments on the difperfive powers of fluids. 
I EXAMINED, by the prifmatic apparatus which has been de- 
fcribed, the optical properties of a great variety of fluid medi- 
ums. It will fuffice to mention the moft remarkable of thefe. 
Many folutions of metals and femi-metals, in different forms, 
were fubjected to trial, and thefe were always found to be more 
difperfive than crown-glafs. The folution of fome falts in wa- 
ter, as for inftance of crude fal ammoniacum, greatly increafes. 
its difperfive power. The marine acid difperfes very confider- 
ably, and this quality increafes with its ftrength. Hence [ 
found the moft difperfive fluids to be thofe in which the marine 
acid and the metals are combined. The chemical preparation « 
galled caufficum antimoniale or butyrum antimonii, in its moft eon- 
centrated ftate, when it has juft attracted fufficient humidity 
to render it fluid, poffeffes the quality of difperfing the rays in 
fuch an aftonifhing degree, that three wedges of crown-glafs are 
neceflary to remove the colour produced by one wedge of this 
° fubftance, 
