Rock Crystals for ISlicromeiers. ^45 



diiiarv course bv the v/cdgo wliich refracts doublv, tliis (Jifference 

 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, consiviting of the vertical and Literal 

 wedges combined, as in the former cas;es, with their acute edges 

 in opposite directions, prodnces an effect perfectly distinct from 

 either of the former con)binations ; for, bv reason of the trans- 

 verse position of their axes of crystallization, the separation of 

 the two images beeonies exactly doubled. The consequence of 

 that position is, that the pencil ordinarily refracted by the first 

 wedge, is refracted extraordinarily bv the second, and that which 

 has been refracted extraordinarily bv the first, suffers a similar 

 intercliange, and is now ordinarily 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 separated .'3 J minutes, just double of that which is effected 

 by either of the ])reccdiug 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 pair of 

 wedges when otherwise combined, which fully e!«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 

 H wedge of 40°, 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 

 o + e, and of the other e + o: they both deviate from their ori- 

 ginal 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 douljlc refraction was not |)crccj)tible. 



" Pour cet cffet," says M. Rochon, " j'employai deux prismes 

 egaux tallies dans le sens le plus favorable a mes vues, ct en Ics 

 presentant dans les deux sens ()p])os('s je trouvai, que dans la 

 premiere disposition la double rclraction n'ctoit pas perceptible, 

 mais, en faisant prendre a mes prismes un seiiLi inverse, la double 

 r(:fraction de cliaque prisme ^toit prescjue doublee." 



Tile correspondence in the effect which I have dcscrii)ed ren- 

 ders this passage from M. Rochon perfectly intelligible; and I 

 hope the directions above given will be sufficient to enable any 

 one to cut a crvslal to the greatest advantage for making this 



Vol. r)(i. No.'iijl. Nov. 1S20. X X sort 



