300 Mr. Herschers Tabic of Refractive Powers, S^c. 



have been sufficient authority for the conchision. Had Sir 

 Isaac made the index of diamond 2.000 in place of 2.439, and 

 that of camphor 1.400 in place of 1.500, he would have ar- 

 rived at the very same result. 



2. When I concluded from my table of refractive powers, 

 that the refractive pozvers of the three simple inflammable sub- 

 stances, viz. DIAMOND, PHOSPHORUS, and SULPHUR, are in the 

 order of their inflammability, I had no other results but a coarse 

 measure of the influence of the two latter substances in altering 

 the focal length of the object-glass of the microscope, (the influr 

 ence being measured by the numbers 4.337 for sulphur, and 

 7.094 for phosphorus,) and these numbers were as good evi- 

 dence of the general principle as if sulphur and phosphorus 

 had been capable of being wrought into the purest transparent 

 prisms, and had their refractive powers determined in relation 

 to the fixed lines in their spectra. 



3. In determining the relation between the index of refrac- 

 tion and the polarizing angle of bodies, the ordinary measures 

 were quite sufficient for the purpose, and on their authority 

 the law has been universally adopted. 



4. When refractive and dispersive powers are measured to 

 discover substances proper for achromatic telescopes and mi- 

 croscopes, a very rude measure is all that is necessary. When 

 I found that sulphuret of carbon possessed most valuable pro- 

 perties, and when I recommended it as a fluid " which might 

 yet be of incalculable service in the construction of optical in- 

 struments," I had taken only the ordinary measures of its ac- 

 tion upon light ; and the practical optician requires no better 

 evidence of the suitableness of the fluid for the construction of 

 fluid object-glasses. 



6. When refractive and dispersive powers are required to 

 enable the practical optician to calculate the curves for an 

 achromatic combination, the greatest accuracy is required ; 

 but in such a case he durst not trust to the measurements of 

 Newton, or Boscovich, orDollond, or Wollaston, or Herschel, 

 or even to those of Fraunhofer, the most accurate of all, be- 

 cause there is no possibility of his commanding the same ma- 

 terials with which their experiments were made. He has no 

 alternative, therefore, but to measure with his own hands the 



