494 



C. II. Warren — A Graduated Sphere. 



a movement possible, the back of the brass plate to which 

 the scale is fastened, was bevelled down to a very thin edge, 

 the bevel beginning about an inch and one-half back from the 

 ends of the scale. The bevelled ends are fastened to a small 

 brass block (see d, fig.) to which is attached a pin that rotates 

 in another metal block which in turn is firmly attached to the 

 horizontal scale (see e, fig.). The end of the vertical scale is 

 set at 0-180° on the horizontal scale. A clip, fitting into a 



Fig. 1. 



Fig. 1. Photograph of an eight-inch graduated sphere for use in work 

 on crystal optics. It is so mounted that it can be rotated rigidly about its 

 vertical axis, or moved in any desired direction upon the cup in which it 

 rests (o). It is provided with three graduated, metal scales, two vertical 

 and one horizontal. One of the vertical scales, (c), can be rotated about the 

 horizontal diameter through an angle of 125° by means of a mounting device 

 shown by the letters d and e. 



shallow notch in the second vertical scale (not shown in the 

 figure) can be used to hold the first scale at the vertical center- 

 point of the sphere, if desired. In practice, however, the 

 writer has not found it necessary to use this clip. 



As the sphere can be rotated rigidly about the vertical axis, 

 or turned in any desired direction, and as one of the vertical 

 scales can be moved through a large angle (125°), it will be 

 evident that there is an entirely adequate freedom of move- 

 ment to make possible the solution, with this sphere, of any of 

 the usual problems met with in crystal optics where spherical 

 projections are used. Great circles, polar to any point, can be 

 located and drawn in with a pencil, and angular values may be 

 determined with great rapidity and ease. For drawing small 



