REFRANGIBIL'lTr of LIGHT. 13 



It 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 1 and if, inftead of this, the coincidence 

 is formed with the moil or lead 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 the* 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 I 

 found the moft difperfive fluids to be thofe in which the marine 

 acid and the metals are combined. The chemical preparation 

 called caujlicum antimoniale or butyrum antimonii, in its moft con- 

 centrated ftate, when it has juft attracted fufEcient humidity 

 to render it fluid, pofTefTes 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, 



