106 Mr Swan on Methods of ascertaining 



since by the motion of the index, the surfaces, of which D C 

 p r are the sections, have been made successively parallel to 

 the same plane, the index must have described an angle equal 

 to p r s, the inclination of these surfaces. But as the index 

 moved from zero, and the angle indicated on the limb of the 

 sextant, is double of that actually described by the index, half 

 the angle indicated will be equal io prs, the angle of the 

 prism, which will thus be ascertained. 



It is obvious that, in these observations, the principal section 

 of the prismm us the adjusted to the plane of reflexion at the 

 mirrors of the sextant. This is easily effected by the hand ; 

 and the correctness of the adjustment will be ascertained by 

 the possibility of making the images coincide accurately. It 

 has also been supposed, for the sake of simplicity, that there 

 is no index error or parallax ; but the existence of either will 

 not alter the determination of the angles. The object observed 

 should be so distant, that the parallax may safely be assumed 

 to be constant during the observations. Its amount, along 

 with the index-error, may then be ascertained by direct ob- 

 servation, and applied as a correction to the angles. 



The sun will be found a convenient object for conducting 

 these experiments ; and as dark glasses are necessary to mode- 

 rate the intensity of the Hght, advantage may be taken of 

 eye-pieces, which transmit only homogeneous rays. Of these, 

 the most convenient is the combination of common red and 

 smalt-blue glasses, recommended by Sir John Herschell,* which 

 will be found to transmit the extreme red rays of the spec- 

 trum of perfectly definite refrangibility. I have also used a 

 fluid eye-piece of the ammoniated sulphate of copper, which 

 absorbs all but the extreme violet rays ; and the flame of 

 a spirit lamp, with a salted wick, in order to obtain yellow 

 rays of definite refrangibility. This light is very convenient 

 for such experiments, and is of the greater importance, as, 

 according to Sir John Herschell, it occupies precisely the same 

 place in the scale of refrangibility as Fraunhofer's dark line 

 D in the solar spectrum. t 



In order to ascertain the index of refraction from the obser- 



* Encyc. Metropolitana, art. Optics, art. 503. t Ibid., art. 436. 



