308 Rev. E. Craig on the Measurement of the 



a half long, having one lens at each extremity ; and the tube is 

 smoothly and accurately fitted, not to screw, but to slip down 

 into the tube of the object-glasses, Fig. 1. 



The lower lens O' receives the image from the object-glasses, 

 and this image is examined by means of the upper lens O. In 

 the focus of the upper lens 0, and lying directly on the plane 

 side of the lower lens O', which is plano-convex, I placed two 

 fine silk threads, crossing each other at right angles. The tube 

 of the eye-piece is slipped down, not screwed down, to its right 

 position in the tube of the object-glasses. It can be turned, 

 therefore, either way freely, without affecting the focal arrange- 

 ment of the microscope, which is regulated by other means. 

 And, by turning the eye-piece round, these fine threads, un- 

 magnified, may be applied to the side of any angle of a highly 

 magnified crystal lying on the field below. 



A brass circle C, about two inches and a half in diameter, 

 with a divided scale on its outer edge, was made to screw fast 

 on to the object-tube T, before the insertion of the eye-piece. 

 And to the edge of the [eye-piece a small index I was fixed, so 

 as to lie close on the circle C, and point to the graduated divi- 

 sion. This index is not a simple point, but a vernier, for in- 

 dicating minutes. 



To measure the angle of a crystal F, it is necessary to bring 

 the point of the angle to the centre of the field of view, where 

 the silk diameters cross each other ; then, by turning the eye- 

 piece round, to bring one of these silk lines to coincide with one 

 side of the angle to be measured, and notice the point of the 



