cutting rock crystal for micrometers. 129 
the separation of the images will be the same, since the 
angles of all the wedges are supposed to be made equal, the 
compound medium will be comprised under parallel surfaces, 
so that a ray ordinarily refracted by both, emerges in its 
original direction ; but since the extraordinary ray is made 
to deviate about 17 minutes from the ordinary course by the 
wedge which refracts doubly, this difference is not corrected 
by the horizontal wedge, so that an object seen through either 
of the combinations L H or V H, appears doubled to the 
amount of 17'. 
The third combination, consisting of the vertical and lateral 
wedges combined, as in the former cases, with their acute 
edges in opposite directions, produces an effect perfectly dis- 
tinct from either of the former combinations ; for by reason 
of the transverse position of their axes of crystallization, the 
separation of the two images becomes exactly doubled. The 
consequence of that position is, that the pencil ordinarily re- 
fracted by the first wedge, is refracted extraordinarily by the 
second, and that which has been refracted extraordinarily 
by the first, suffers a similar interchange, and is now ordi- 
narily refracted, so that neither of the divided pencils returns 
to its true place; and since one falls as much short of the 
mean as the other exceeds the truth, they emerge ultimately 
separated twice the usual difference between the ordinary and 
extraordinary refractions, and thus present two images sepa- 
rated 34 minutes, just double of that which is effected by 
either of the preceding combinations. 
Though it could scarcely be doubted that this is essentially 
the construction which was employed by M. Rochon, there 
is an additional circumstance concerning the effect of such a 
mdcccxx. S 
