XOTES. 



467 



passing through a hole at H, made with a fine needle in a piece of lead or 

 a card, and received on a screen S S'. When a hair, or a small slip of 



Fig. 56. 



card h h' , about the 30th of an inch in breadth, is held in the beam, the 

 rays bend round on each side of it, and, arriving at the screen in different 

 states of vibration, interfere and form a series of coloured fringes on each 

 side of a central white band m. When a piece of card is interposed at C, 

 so as to intercept the light which passes on one side of the hair, the 

 coloured fringes vanish. When homogeneous light is used, the fringes are 

 broadest in red, and become narrower for each colour of the spectrum pro- 

 gressively to the violet, which gives the narrowest and most crowded 

 fringes. These very elegant experiments are due to Dr. Thomas Young. 



NOTE 199, pp. 171, 200. Fig. 58 shows Newton's rings, of which 

 there are seven, formed by screwing two lenses of 



glass together. Provided the incident light be 

 white, they always succeed each other in the fol- 

 lowing order : 



1st ring, or 1st order of colours: Black, very 

 faint blue, brilliant white, yellow, orange, red. 



2nd ring : Dark purple, or rather violet, blue, 

 A very imperfect yellow green, vivid yellow, 

 crimson red. 



3rd ring : Purple, blue, rich grass green, fine 

 yellow, pink, crimson. 



4th ring : Dull blueish green, pale yellowish pink, red. 



Fig. 58. 



