1835.] Composition of White Light. 359 



rays,* of as many different colours making their progress 

 in three different directions, must ultimately produce, when 

 any part of it is intercepted, a distinct shadow of the inter- 

 cepting object in each colour : during its progress, how- 

 ever, and previous to the complete developement of the 

 different colours, the three shadows will remain united, and 

 exhibit, with a continually increasing breadth, a variety of 

 coloured fringes ; some of them simple, and others formed 

 by the intersection of two colours. 



The action of the edges of the interposed bodies has 

 nothing to do with the production of this class of coloured 

 fringes : these edges act merely as boundaries, that allow 

 certain rays to pass, which, by their intersection at this 

 point, form white light ; hut which, in their further pro- 

 gress, require for this purpose a succession of other similar 

 rays ; and these being intercepted by the obstructing body, 

 they make their appearance in their proper colours, and in 

 their own independent direction. The fringes in refracted 

 white light, while they continue distinct, are violet and blue 

 on one side of the shadow, and yellow and red on the other ; 

 and by observing their progress, whatever may be their 

 breadth, we shall find that the two sides ar-e complementary 

 to each other ; so that if one were moved towards the other, 

 exactly the space occupied by the intercepting object, the 

 violet would fall upon the yellow, and the blue upon the 

 red, and again produce an uniform surface of white light. 



The following simple experiment will prove this : Place 

 two white cards upon a black surface, with their edges 

 parallel, and about an inch from each other : upon viewing 

 the cards thus arranged, through a prism, with the refract- 

 ing angle downward, and held at a short distance from them, 

 the fringes upon the upper card will be blue and violet, and 

 upon thfe lower, red and yellow ; the distance of the prism 

 may be easily regulated so as to bring the violet of the 

 upper card close to the red of the under card, but without 

 interfering with it : if we now fix the prism in this position, 

 and with a pair of compasses, opened to the distance of the 

 cards from each other, measure the space from the centre 

 of the blue to the centre of the red, and from the centre of 

 the violet to the centre of the yellow, we shall find that the 



\ See note p. 358. 



