510 Mr. J. T, Cooper's improvements in Dr. Wollaston's 



quiring extremely small quantities of the matter to be sub- 

 jected to experiment, was an additional recommendation for 

 its adoption in practice, but it has never been generally brought 

 into use ; and the cause which in my opinion has operated 

 more than any other to prevent its more extended employ- 

 ment in the laboratory of the chemist, is the limited extent of 

 substances which, in the form he gave it to the world, could 

 by its means have their indices of refraction determined with 

 accuracy. It is with the view of rendering the method of 

 Dr. Wollaston more generally known, and of putting the 

 Society into possession of a knowledge of some alteration in 

 the construction of the instrument, by which it is rendered of 

 a more extended application, that I have ventured to call the 

 attention of the Society to this subject. 



Instead of employing only one species of glass as recom- 

 mended by Dr. Wollaston, I make use of several, each of 

 which is suited to the nature of the substance under examina- 

 tion, and that species of glass is selected for the purpose, which, 

 when the substance to be examined is applied to the base of 

 the prism, gives with the subject so applied, when viewed in 

 its position, neither too acute or too obtuse an incidence ; for 

 it is at extreme incidences, as far as I have been able to ob- 

 serve by the means here employed, that erroneous results are 

 liable to be obtained. The glass prisms which I am in the 

 habit of using have respectively a refractive index for Fraun- 

 hofer's ray b of 



1*516, which is ordinary plate glass. 



1*583, which is common flint glass. 



1*635, a very heavy flint glass, which I made some years 

 ago for optical purposes. 



1*816, Faraday's borate of lead glass. 



Now in order that these different prisms may be used, a 

 modification of the apparatus as originally proposed by Dr. 



g d e 



Wollaston is requisite, or otherwise each prism will require a 

 separate and distinct instrument ; but to accomplish this ob- 

 ject with but one instrument, I have changed the position of 

 the indicator from the longer of the bars to the shorter, and 

 constructed the longest bar a in such a manner that it is ca- 

 pable of being extended in length from 15*16 to 18*16 inches, 



