469 
of Edinburgh, Session 1870 - 71 . 
which proved very successful. I then published a paper on it in 
the Phil. Mag. for 1834, vol. iv. p. 289, from which I must ask 
leave to make an extract, as a necessary introduction to what I 
wish to say about it on the present occasion. 
My paper begins by quoting the testimony of the German writer 
to the merits of the instrument, and continues thus : — 
“Poggendorff then goes on to say, that as Mr Nicol had not 
attempted to explain the operation of the instrument, he would 
endeavour to do so, in which, however, I cannot say that I think 
he has been entirely successful. Now, it will he observed that the 
inventor attributed the fact of the instrument’s producing only one 
image to a great 1 divergency ’ which it causes in the images, 
throwing one of them aside out of the field of view. The German 
writer follows the same idea, but adds, that in his opinion such 
divergency is caused by the Canada balsam, whose index of 
refraction being 1-549, is intermediate between that of the ordinary 
ray 1 654 and that of the extraordinary ray 1*483, which circum- 
stance will (in his opinion) account for the rays being 1 thrown 
opposite ways.’ He adds, that any one 1 who was not afraid of the 
trouble ’ might easily calculate the path of both rays, a remark 
which shows that his idea was that they were both transmitted, 
and diverging from each other. But I find that this great diver- 
gency does not, in point of fact, exist, for by inclining the 
instrument a position may be found in which both images are 
seen, and they are then very little separated, not more so than they 
were by the same piece of spar before its bisection and cementation. 
On gradually altering the position of the instrument, the second 
image is not seen to move away from the first ; but at a certain 
moment it vanishes suddenly without leaving the smallest trace of 
its existence behind. Having thus described the appearances as 
I have found them, I will give an explanation of them, which I 
hope will be more satisfactory. As long as the rays composing the 
images are incident upon the Canada balsam at moderate obli- 
quities, it cannot exert any particular discriminating action upon 
them. But when the obliquity reaches a certain point, one of the 
images sulfers total internal reflexion, because the Canada balsam 
is (with regard to that image) a less refractive medium than calc 
spar. But with regard to the other image, it is at the same 
3 R 
VOL. VII. 
