16 Prof. W. G. Adams on a new Polariscope. 



should be so placed that 0, the centre of curvature of the two 

 curved surfaces of the lenses, coincides with the focus to which 

 the rays converge after they come out of the first system of 

 lenses. The rays will then pass through this central piece with- 

 out having their direction altered, since the crystal will refract 

 the light nearly to the same extent as glass, and the light will 

 pass nearly as if it passed through the centre of a sphere of glass. 



After passing through this central piece, the rays diverge from 

 the common centre of curvature and fall on a plano-convex lens, 

 F, 1 inch in diameter, and placed about f of an inch from the 

 common centre of curvature. Its focal length should be 1J inch. 

 At a distance of half an inch from this lens is another lens, G, 1| 

 inch in diameter and If inch focus, so as to make the rays again 

 parallel. Then at a distance of 3 inches is placed a lens, H, 

 whose focal length is rather less than 3 inches and whose dia- 

 meter is 1 J inch ; and above this the Nicol's prism, K L, with 

 any other apparatus, such as the beautiful arrangements of Mr. 

 Spottiswoode for showing the effects of quartz and other crystals 

 on polarized light. At the principal focus of the last lens, H, 

 should be placed crossed spider-lines ; and for accurate mea- 

 surement a simple lens or eyepiece should be added above the 

 NicoFs prism. 



The object of receiving the light from the mirror on a double- 

 concave lens is to avoid aberration and the consequent loss of 

 light ; but this involves the use of a large piece of tourmaline. 

 The difficulty is got over in existing polariscopes by employing 

 two convex lenses, causing the rays to cross the axis very near 

 the polarizing tourmaline, and then making them parallel by 

 means of a second convex lens. The central piece may be added 

 to any polariscope provided there is sufficient room between the 

 two systems of lenses to admit the crystal and two lenses, M, N, 

 and to allow of motion about the axis through the common 

 centre of curvature of the surfaces of these two lenses. The two 

 parts, BCDE and F G H, should be so arranged that when the 

 crystal alone is placed at the rings are best seen ; then the 

 crystal should be placed in the box between the two lenses M 

 and N in the bottom and in the cover of the box. This box 

 should be supported on two wires, P and Q, forming an axis 

 passing through 0, and should be large enough to contain the 

 cork in which the crystal is placed. The axis is supported 

 in a brass tube, which forms one piece with another brass tube 

 which fits on the piece BCDE. A semicircular disk of brass 

 with its arc graduated is fixed on the large brass tube about 

 the axis P as a centre; and on this axis is an index for read- 

 ing-off the angle through which the axis is turned. On turn- 

 ing the box about the axis, no change is produced in the rays 



