446 



Mr. W. Crookes on a New Arrangement of [May 27, 



through the object-glass, to pass freely up the tube K. A pin at M en- 

 ables the prisms to be thrown either in or out of action by a movement of 

 the finger. As the prisms are close above the object-glass, the usual sli- 

 ding box, carrying the binocular prism and the Nicol's prism (shown at N), 

 may be employed as usual, and the spectrum of any substance may thus 

 be examined by both eyes simultaneously, either by ordinary light, or when 

 it is under the influence of polarized light. The insertion of the prism-box 

 between the object-glass and the body of the microscope does not interfere 

 with the working of the instrument in the ordinary manner. The length of 

 the tube is increased 1 or 2 inches, and a little additional rackwork may in 

 some instruments be necessary when using object-glasses of low power. 

 The stereoscopic effect when the Wenham prism is put into action does 

 not appear to be interfered with. 



Fig. 2. 



For ordinary work both these additions may be kept attached to the mi- 

 croscope, the prisms being pushed to the side of the prism-box, and the 

 large aperture D being brought into the centre of the substage. When it 

 is desired to examine the spectrum of any portion of an object in the field 



