125 



glass, so that a luminous object seen through them shall appear in 

 its true place by ordinary refraction, accompanied by a second image 

 produced by extraordinary refraction. 



In consequence of the dispersion of colours which occurs in em- 

 ploying different substances, such a combination is not suited for the 

 micrometer invented by Abbe" Rochon ; but it is not difficult to ob- 

 tain such a section of rock-crystal as may be substituted for the glass 

 wedge, so that the pencil of light shall be colourless without di- 

 minishing the separation of the images. But since the degree to 

 which the double refraction of rock crystal separates the two por- 

 tions of a beam of light transmitted through it, is sometimes not suf- 

 ficiently great, it becomes desirable to increase it ; and though the 

 means of effecting this have not been described, the author proceeds 

 to explain the method that he has found advantageous, and which he 

 regards the same as that of M. Rochon. 



The author then describes three modes of cutting wedges of rock 

 crystal, the axis of crystallization being differently placed in each. 

 In the first, or horizontal wedge, the axis is at right angles to the 

 first surface. In the second, or lateral wedge, the axis is in the first 

 surface and parallel to its acute edge. In the third, or vertical wedge, 

 the axis is also in the first surface, but at right angles to the acute 

 edge. An object seen through the first wedge, in the direction of the 

 axis, does not appear double ; but in the others the transmitted rays 

 pass at right angles to the axis, and they each produce two images. 



By placing two of these wedges together, with their acute edges 

 in opposite directions, there are obviously three modes in which they 

 may be combined in pairs, represented by LH, VH, and VL. In 

 the two first cases, the separation of the images will be the same, 

 and an object seen through the combination appears double to the 

 amount of 17'; but the third produces a distinct effect ; for, by 

 reason of the transverse position of the axes of crystallization, the 

 separation of the two images becomes exactly doubled. The pencil 

 ordinarily refracted by the first wedge is refracted extraordinarily by 

 the second, and vice versa, so that neither of the divided pencils re- 

 turns to its true place ; and since one falls as much short of the 

 mean as the other exceeds the truth, they are ultimately separated 

 twice the usual distance between the ordinary and extraordinary re- 

 fractions, and thus the images are separated 34'. This, the author 

 says, it can scarcety be doubted is essentially the construction em- 

 ployed by M. Rpchon. This paper is concluded by some further 

 directions respecting the mode of cutting and arranging the prisms 

 for the above purpose. 



On a New Principle of constructing Ships in the Mercantile Navy. By 

 Sir Robert Seppings, F.R.S. Read March 9, 1820. [Phil. Trans. 

 1820, p. 133.] 



In the present mode of constructing the ribs of English merchant 

 ships, only half the timbers are united, so as to constitute any part 



