weight: a new crystal-grinding goniometer 



37 



and accurately fitted. There are four essential parts, as indicated 

 in figures 1, 2, and 3: (a) telescope (fig. 3a) ; (b) two-circle theodo- 

 lite arrangement for holding and measuring crystals (fig. 2); 

 (c) grinding plate (fig. 2) ; (d) optical system of an axial angle 

 apparatus mounted on a special tripod (fig. 3b). A brief de- 

 scription of these parts will now be given. 



The telescope (fig. 3a) is fitted with an autocollimating device^ 

 which consists of a strip of thin cover-glass of the shape indicated 

 in figure 3e; this is placed 

 in the rear focal plane of 

 the telescope objective 0. 

 Light from a small tung- 

 sten lamp is sent through 

 the plate to the inclined 

 edge of the cover-glass, 

 where it is totally reflect- 

 ed and passes thence 

 through the objective 

 and prism to the crystal 

 plate, whence it is re- 

 flected back through the 

 lens system and appears 

 then, when viewed 

 through the eyepiece, as 

 a sharp bright line in the 

 dark field. Settings are 

 made by covering the bright line by the reflecting edge of the glass 

 slip, so that the field becomes practically dark. A transverse 

 etched line across the center of the reflecting edge serves as 

 reference point for the setting. In place of the transverse line two 

 short cover glasses, separated slightly and so mounted that their 

 reflecting edges form a straight line, have been used to advantage. 

 The images due to reflection from the faces of the telescope prism 

 {X, fig. 3a) are easily eliminated by mounting the prism in shghtly 

 tilted position so that the rays reflected from its faces reach the 

 image plane outside the field of the positive eyepiece E. The 



3 F. E. Wright, this Journal, 4: 235. 1913. 



Fig. 3 



