376 MR SWAN’S EXPERIMENTS ON THE 
Some time ago, Mr Wixx1am Nicot 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 
was 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 
in 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 
sin } (0+a) 
6 
ate where 6 is the angle of the prism, and a the mi- 
from the formula up = 
nimum deviation of the refracted rays. 
The theodolite I used in this investigation is made by Troucuton. 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 with 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 Joun ApIE. 
A rod of well-seasoned mahogany, fitted to the Ys of the theodolite, was furnished 
at one end with temporary 1's, 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 Edinburgh New Philosophical Journal, January 1844. 

