376 MR SWAN'S EXPERIMENTS ON THE 



Some time ago, Mr William Nicol of Edinburgh, whose skill in cutting and 

 polishing Iceland spar is well known, requested me to undertake the examination 

 of the ordinary refraction of several prisms of Iceland spar, with which he had 

 the kindness to entrust me, and which he had cut so that the transmitted light 

 is inclined at various angles to the axis. The refractive power of these prisms 

 Avas examined by means of an instrument devised by me for facilitating such in- 

 quiries, and described in the Transactions of the Royal Scottish Society of Arts 

 for 1844, p. 293.* It will be sufficient here to explain that the prism is mounted 

 ill front of the telescope of a theodolite, with plates of sextant glass in accurate 

 contact with its faces. The deviation of the refracted rays is then measured as 

 in Fraunhofer's method of determining refractive powers ; and the refracting 

 angle of the prism is ascertained by measuring the deviation of rays that have 

 suffered two reflexions at the surfaces of the sextant glasses. The prism being 

 placed in its position of minimum deviation, the index of refraction is ascertained 



from the formula fx = ^'" :^ \ t "•^ where 6 is the angle of the prism, and a the mi- 

 nimum deviation of the refracted rays. 



The theodolite I used in this investigation is made by Troughton. The 

 horizontal limb, measuring 6-5 inches in diameter, is furnished with two verniers 

 reading 20', and the telescope magnifies twelve times. As I had not the means 

 of observing an object at a greater distance than 40 feet, and as the correction for 

 parallax due to the distance of the prism from the centre of the theodolite could 

 not be ascertained Avith sufficient accuracy, owing to the difficulty of finding 

 the exact position of the pencil of incident rays, I determined to adopt a method 

 for avoiding this correction. 



This consisted partly in mounting the prism over the centre of the theodolite 

 by means of a simple and ingenious arrangement suggested by Mr John Adie. 

 A rod of well-seasoned maht)gany, fitted to the Ys of the theodolite, was furnished 

 at one end with temporary 1\ placed so as to shift the telescope out from the 

 centre of the instrument ; while, at the other, it carried a counterpoise to the 

 weight of the telescope. To this I added stays of wire passing from the ends of 

 the rod to the extremities of the horizontal axis of the theodolite, which were 

 tightened by means of screws so as to prevent any lateral shake. The whole 

 apparatus was mounted on a very firm portable tripod, and was sufficiently stable. 



But although the prism, from its position at the centre of the instrument, 

 did not suffer any material displacement on turning round the telescope, it 

 was still desirable to get rid of any remaining uncertainty as to the direction of 

 the incident light. The method I devised for effecting this object, was to use a 

 collimator so as to obtain a beam of sensibly parallel rays, and thus to place 



* Also in the Kdinburgh New Philosophical Journal, January 1844. 



