14 Prof, W. G. Adams on a new Polaiiscope. 



These advantages have been obtained by modifying the posi- 

 tions and focal lengths of the lenses usually employed in table 

 polariscopes, so that the rings of a crystal are best seen when 

 there is a space of 1^ inch between the two lenses, one on either 

 side of the crystal. Into this space is introduced a central piece, 

 consisting of a circular box with deep plano-convex lenses fixed, 

 one in the bottom and the other in the top of the box, in such a 

 position that their curved surfaces have a common centre of cur- 

 vature, with their flat faces turned towards one another and en- 

 closing the crystal between them. The box can turn about an 

 axis passing through the common centre of curvature. 



The form and position of Mirror and Lenses for the Polariscope. 



A is a concave mirror about 1J inch in diameter, such as is 

 ordinarily employed for illuminating microscopes, and mounted 

 in the same manner. A double-concave lens, B, 1 inch in dia- 

 meter, is placed so as to have its focus at the same point as the 

 principal focus of the mirror. The rays coming from the mirror 

 will then be parallel after passing through the lens. To dimi- 

 nish aberration as much as possible, the radius of curvature of 

 the first face of the lens should be about six times the radius of 

 its second face. The parallel rays then fall on a tourmaline or 

 other polarizer, C, the diameter of which should be nearly equal 

 to that of the lens. They then fall on a double-convex crossed 

 lens D, the first face of the lens having the greatest curvature, 

 so as to diminish the aberration as much as possible. This lens 

 is 1 inch in diameter and 1 £ inch focal length. 



At a distance of J of an inch from this lens is placed a convexo- 

 plane lens E, f of an inch in diameter and 1 inch focal length . 



These three lenses, B, D, and E, may be fixed in the same 

 piece ; and with the above arrangement, rays which are parallel 

 before falling on the mirror will be brought to a focus at a distance 

 of half an inch on the outside of the last lens E. The rays then 

 fall on the central piece M N, consisting of two plano-convex 

 lenses, which are nearly hemispheres, enclosing the crystal be- 

 tween them. The first of these lenses, M, is 5 millims. in thick- 

 ness, and the radius of its spherical surface is 6*4 millims., or 

 J of an inch ; the other lens, N, is 7 millims. in thickness, and 

 the radius of its spherical surface is 9*6 millims., or -J of an inch ; 

 and they should be so placed that the two centres of curvature 

 are accurately at the same point 0. The distance between them 

 will then be 4 millims., which is very nearly the thickness of 

 most specimens of crystals. If lenses w T ith the same curvatures 

 but of thicknesses 4 \ and 6^ millims. respectively be used in 

 place of M and N, there will be a breadth of 5 millims. between 

 them when they are in the best position. This central piece 



