the Index of Refraction. 106 



T Urepresents partof the telescope of the sextant, and A B, CD, 

 sections of the horizon and index-glasses, by a plane per- 

 pendicular to their surfaces. Let the prism of which p r a 

 (fig. 1) is a section, be applied, with its surface slightly mois- 

 tened, to the horizon-glass, and it will adhere in any required 

 position by the cohesive attraction of the film of fluid be- 

 tween it and the glass. The telescope can now receive 

 simultaneously rays, labc, passing through the horizon -glass, 

 and refracted by the prism,* and hhkm, passing under the 

 prism, and reflected from the index and horizon glasses. The 

 images of a distant luminous object, formed by the refracted 

 and reflected rays, may therefore be brought into coincidence 

 at the focus of the eye-piece, as in the ordinary manner of 

 using the sextant ; and from the nature of the instrument, the 

 angle indicated will be the inclination of the final direction 

 g c, of the refracted rays to L A, the direction of the rays inci- 

 dent on the index-glass. Now, if the luminous object be so 

 distant, that the parallax, due to the distance between the 

 horizon and index-glasses, may be neglected, the inclination 

 of ^ c to L ^ will be sensibly the same as gel, the deviation 

 of the refracted rays. Since, therefore, the angle indicated 

 by the sextant is the inclination of ^c to L h, the deviation of 

 the refracted rays is determined. 



If the prism be now placed upon the index-glass , (fig. 2) 

 in the position pr Sy its surface p r will form a new index- 

 glass, inclined to the proper index-glass C D, at the angle 

 pr s, which is to be determined. Let the image of a luminous 

 object, seen by rays reflected from the index-glass, be made 

 to coincide with the image of the same object seen directly ; 

 and if there be no parallax or index error, the surface D' O of 

 the index-glass will be parallel to the surface of the horizon- 

 glass, and the index will be at zero. Again, let the image of 

 the same object, formed by rays reflected from the surface 

 p r, be made to coincide with the object seen directly, and 

 the surface p r will be parallel to the horizon-glass. Now, 



• In this, and also in the other figures, as the rays do not suflGer any ultimate 

 change of direction in passing through the parallel plates of glass, in order to 

 simplify the diagrams, they are represented as suffering no refraction by these 

 plates. 



