130 Dr. Wollaston on the methods of 
pair of wedges when otherwise combined, which fully es- 
tablishes the identity of the method here proposed with his. 
If the two wedges be placed with their edges together, so as 
to form by their union a wedge of 4,0°, the consequence is, that 
though a pencil of light is in fact divided into two parts by 
the first wedge, both parts in the end emerge together; the 
refraction of one being 0 - \-e, and of the other e -j- 0 : they 
both deviate from their original direction by exactly the same 
quantity, and present only a single image of the luminous 
object ; but it is coloured, as usual, in proportion to the amount 
of deviation occasioned by the sum of the wedges. This, 
without doubt, is the first of two opposite directions mentioned 
by M. Rochon, in which he says the double refraction was 
not perceptible. 
“ Pour cet effet,” says M. Rochon, “j’employai deux 
“ prismes egaux tailles dans le sens le plus favorable a mes 
“ vues, et en les presentant dans les deux sens opposes je 
<c trouvai, que dans la premiere disposition la double refrac- 
“ tion n’etoit pas perceptible, mais, en faisant prendre a mes 
“ prismes un sens inverse, la double refraction de chaque 
“ prisme etoit presque doublee.” 
The correspondence in the effect which I have described, 
renders this passage from M. Rochon perfectly intelligible ; 
and I hope the directions above given will be sufficient to 
enable any one to cut a crystal to the greatest advantage for 
making this sort of micrometer. But it must be observed, 
that in attempting such a construction, great nicety is requi- 
site, not only in cutting the wedges so that the refraction in 
each shall take place at right angles to the axis, but also in 
cementing them together, so that the axes of the two wedges 
