469 



Fig. 62. 



6th order : Very faint green, very faint red. 

 7th order : A trace of green and red. 



NOTE 202, p. 175. Let L L', fig. 61, be the section of a lens placed in 

 a window-shutter, through which a very small beam of light S L L' passes 

 into a dark room, and comes to a focus in F. If the edge of a knife K N 

 be held in the beam, the rays bend away from it in hyperbolic curves 

 K r, K r', &c., instead of coming directly to the screen in the straight line 

 K E, which is the boundary of the shadow. As these bending rays arrive 

 at the screen in different states of undulation, they interfere, and form a 

 series of coloured fringes, r r', &c., along the edge of the shadow K E S N of 

 the knife. The fringes vary in breadth with the relative distances of the 

 knife-edge and screen from F. 



NOTE 203, p. 177. Fig. 43 represents the phenomena in question, 

 where S S is the surface, and I the centre of incident waves. The reflected 

 waves are the dark lines returning towards I, which are the same as if 

 they had originated in C on the other side of the surface. 



NOTE 204, p. 180. Fig. 62 represents a prismatic crystal of tour- 

 maline, whose axis is A X. The slices that are used for polarising light 

 are cut parallel to A X. 



NOTE 205, p. 181. Double refraction. If a pencil of light Rr, fig. 63, 

 falls upon a rhombohedron of Iceland spar A B X C, it is separated into 

 two equal pencils of light at r, which are refracted in the directions 

 r O, r E : when these arrive at and E they are again refracted, and pass 

 into the air in the directions o, E o, parallel to one another and to the 



