THE AMICI PRISM. 



73 



used in the substage, as shown in Fig. 51, or better 

 perhaps upon a separate stand, so that it is entirely inde- 

 pendent of the microscope or stage, Fig. 52. 



In using this prism it is set beneath the stage at such a 

 distance from the axis of the microscope as to furnish rays 

 of sufficient obliquity, and in 

 order to produce the best 

 effect the stage should be 

 capable of rotating round the 

 fixed luminous rays, as with 

 line objects, such as the silici- 

 ous valves of diatoms, the 

 Navicula rhomboides, for in- 

 stance, there is nearly always 

 one position in which the 

 markings are shown to the 

 best advantage. 



Colonel Woodward's prism 

 has been described upon p. 246, 

 vol. i. of the 'Journal of the 

 Royal Microscopical Society.' 

 It consists of a right-angle 

 prism of crown glass, the long 

 side of which measures J inch 

 by J inch wide ; it is cemented 

 into a base-piece of brass, 

 and supported on a stout 

 steel rod three or four inches long. Colonel Woodward 

 tells us that the whole apparatus ought not to cost 

 more than three or four shillings, but we are afraid he 

 has not consulted the optician in this matter. To use 

 it, the steel rod is slipped into a dark-well holder, and 

 putting a drop of oil on the upper face of the prism, it 

 is placed in contact with the under surface of the slide. 



FIG. 52. 



