OK OPTICAL INSTRUMENTS. 421 



intensities are inversely as the squares of their distances. Count Rumford's 

 photometer performs this very conveniently, by casting two shadows of a 

 given object near each other, on the same surface, the lights being removed 

 to such distances that the shadows appear equally dark. (Plate XXVII 

 Fig. 387, 388.) 



For determining the refractive density of solids, it has been usual to form 

 them into a prism, and to measure the angular deviations which they 

 produce ; and for fluids, to inclose them either in a hollow prism, or between 

 two meniscus lenses, and to measure the angular deviations produced by the 

 prisms, and the focal distances of the lenses. But in most cases. Dr. Wol- 

 laston's apparatus is far preferable to both these methods: it is arranged for 

 ascertaining the angle at which light, moving within a certain dense trans- 

 parent substance, begins to be totally reflected from the common surface of 

 that substance and the solid or fluid which is to be examined. Thus, if we 

 first measure the angle, at which light begins to be totally reflected from the 

 posterior surface of a prism of glass, in contact with air, we may readily 

 determine its refractive power; and then, having caused a drop of a fluid 

 to adhere to that surface, or iixed a solid to it by a small portion of some 

 fluid denser than itself, we may observe, as we turn the prism round its 

 axis, at what angle the drop or spot begins to disappear, and may thence 

 calculate the refractive density of the substance; and even without actual 

 measurement of the angle, we may readily compare the disappearance of the 

 drop or spot with that of others placed near it, of which the properties are 

 known. Dr. Wollaston has, however, rendered the process still easier and 

 "more simple, by employing a rectangular prism of glass, with sights fixed to 

 a jointed frame, of such a construction as to enable him to read oft^ by a 

 vernier, without any calculation, the index of the refractive power of 

 any substance less dense than glass. (Plate XXVII. Fig. 3S9.) 



All instruments strictly optical are employed for forming an image of an 

 external object: the simplest are mirrors and lenses, which form a single 

 image only, either actual or virtual, and sometimes depict it on a surface 

 calculated for receiving and exhibiting it. Other instruments repeat the 

 image once or more under several forms, in general enlarging it continu- 



