242 



USE OF THE MICROSCOPE. 



of the circle the colours will be seen to change alternately 

 from red to green, and from green to red, fig. 155, a' H c' d'. 

 If the brass be slid along so as to bring the two largest aper- 

 tures into focus, the images will overlap, and where they do 

 so white light will be produced, as shown in fig. 154, 6' c'. 



rig. 155. 



The experiments may be varied by employing two double 

 refracting crystals, placed one over the other, and by removing 

 the NicoFs prism, but retaining the brass plate on the stage ; 

 two distinct images will then be seen, but at twice the original 

 distance from each other, as in fig. 156, a. If the crystal 



Fig. 156. 



nearest the eye be turned from left to right, two faint images 

 will appear ; continuing the revolution, the four images will 

 be equally luminous, and when the crystal has been turned 

 round 90°, there will be only two images of equal brightness ; 



