Refractive powers of several Bodies. 299 



^jefore us oils purchased in 1810, and which have no resem- 

 blance to similar oils which are now obtained under the same 

 name. Some of them, indeed, have deposited groups of crys- 

 tals, and consequently they must have become almost new 

 substances. 



In the examination of gums, and such like hard solids, their 

 refractive power depends on the degree of induration which 

 they possess, and this will depend on th6 place where they have 

 been kept ; so that the same gum will give different results at 

 different times. 



In the case of minerals, every thing depends on the nature of 

 the specimens which the observer can command. I have often 

 been compelled to measure both refractive and dispersive pow- 

 .ers with fragments not bigger than a pin's head, and with crystals 

 that almost escaped unassisted vision. At -other times, I have 

 been obliged to work with specimens which I was not allowed 

 to cut, and in this case, when the natural faces were imper- 

 fect, there was no resource but to take a mean of the inclina- 

 tions of different parts of the faces, and a mean of the angle of 

 deviation, or, what is sometimes better, to mask with an opaque 

 cement all the imperfect portions of the surface, excepting 

 those which had the best polish, and the most uniform incli- 

 nation. 



On other occasions, it was necessary to keep the crystal in 

 its matrix, and to resort to the most troublesome methods of gain- 

 ing a measure of its refractive or dispersive powers. All this 

 labour, however, and none but those who have been exposed 

 to it can form an idea of it, was not undergone to obtain mere- 

 ly a measure of refractive indices, or of dispersive actions, but 

 for purposes much more important, and more interesting to 

 the observer. This circumstance leads us to consider the va- 

 rious objects for which such measurements are taken. 



1. When refractive or dispersive powers are measured to 

 determine physical or chemical relations, numerical accuracy 

 is of no importance. When Sir Isaac Newton, for example, 

 deduced from his refractive indices of camphor, olive oil, lin- 

 seed oil, spirit of turpentine, amber, and the diamond, his beau- 

 tiful conclusion that the latter was probably an inflammable 

 substance coagulated, the measures of the merest tyro would 



