112 On Methods of asceriamin^ the Index of Refraction. 



It is obvious, that this method of determining refractive 

 powers is particularly well adapted for fluid substances. The 

 fluid may either be contained in a hollow prism made of paral- 

 lel plate glass, or a few drops may be retained by capillary 

 attraction between the glasses of the instrument ; in either 

 case, the angle of the fluid prism will be ascertained in the 

 manner already shown. 



From a considerable number of observations, I have found 

 that the method w^hich has now been described, affords con- 

 siderable facilities in determining refractive powers. The 

 construction of the apparatus makes the adjustment of the 

 surfaces of the prism quite independent of each other. For 

 the rings E F, G H (fig. 6), and the face of the prism in con- 

 tact with the glass A B, remain constantly perpendicular to 

 the divided limb ; and the face in contact with the glass C D, 

 is directly adjusted by the motion of the ring E F. 



The application of glass-plates to the surfaces of the 

 prism also greatly facilitates the process, inasmuch as it ren- 

 ders it unnecessary to have the faces of the prism so highly 

 polished as would otherwise be indispensably requisite. In 

 fact, it is generally sufficient to grind the surfaces smooth 

 with fine emery, when the fluid by which the prism is re- 

 tained between the glasses will render it perfectly diaphanous. 

 Sir David Brewster, who seems to have been the first to avail 

 himself of this method of making prisms, observes, that, in 

 measuring the refractive and dispersive powers of bodies that 

 are incapable of receiving a good polish, " by cementing upon 

 the two refracting planes pieces of parallel glass with a fluid 

 of nearly the same refractive density, substances like horn, 

 rock salt, and several of the gums, may be rendered perfectly 

 transparent."* 



In alluding to the circumstance, that the prism is included 

 between two plates of glass, by whose inclination its refracting 

 angle is ascertained, I am led to anticipate some objections 

 to the accuracy of the process that may suggest themselves to 

 the reader. 



It may be thought, that not only will the refractive power 

 of the glass-plates vitiate the result, but also that a source of 



* Treatise on New Philosophical Instruments, p. 279. 



