the Index of Refraction, 103 



In later times, the chief accessions to our knowledge of re- 

 fractive and dispersive powers are due to Sir David Brewster 

 and Dr WoUaston ; but although the labours of these philoso- 

 phers have greatly extended our acquaintance with such sub- 

 jects, much may yet be done to verify and render more exact 

 the results which have been obtained. Sir John Herschel ob- 

 serves, that " an accurate re-examination of the refractive and 

 dispersive powers of natural bodies of strictly determinate 

 chemical composition and identifiable nature, although, doubt- 

 less, a task of great labour and extent, would be a most val- 

 uable present to optical science;" and again, that " Fraunhofer's 

 researches shew to what a degree of refinement the subject may 

 be carried, as well as the important practical results to which 

 it may be applied."* 



It would be superfluous to adduce any illustrations in proof 

 of the correctness of these views, or of the importance of the 

 inquiries to which they refer, in addition to the example of 

 the justly celebrated Fraunhofer, whose admirable researches 

 affbrd one of the finest instances of the connexion which sub- 

 sists between the extension of science, and the improvement 

 of the arts. 



If, then, it be desirable not only to extend our knowledge, 

 but also to render it more accurate, we shall best promote 

 these objects by facilitating our methods of observation ; and, 

 although we are already possessed of instruments by means o^ 

 which tolerably accurate results may be obtained with great 

 facility, we may yet find room for improvement in simplifying 

 our more exact, but at the same time, more difficult pro- 

 cesses. 



Among the latter class of methods, must be ranked the 

 most direct and obvious process of finding the index of refrac- 

 tion, which consists in forming a prism of the substance whose 

 refractive power is to be determined, and then ascertaining 

 the inclination of its refracting surfaces, and the deviation of 

 a ray of light transmitted through them. In order to find 

 the index of refraction from such observations, it is necessary 

 to place the prism, with its face inclined, at a determinate angle 



* Encyclopedia Metropolitana, " Optics," p. 673, 



