290 ON LIGHT. 



spectrum be made to traverse to and fro on the illumin- 

 ating screen, the rings will appear to open and close in 

 an exceedingly beautiful manner, undergoing at the same 

 time a corresponding change of colour. 



(73.) The composite nature of the rings, as seen in 

 white light, is now abundantly clear. White light is a mix- 

 ture of all the prismatic rays, and the set of rings seen in 

 such licrht is of course a mixtiwe of the several individual 

 sets {concentric^ hut diffciingby a regular gradation of size), 

 of all the several coloured elements of which white light 

 consists. Imagine a painter wdio could " dip in the 

 rainbow" and lay on, one after another, on the same 

 paper and wnth the same centre, such a series of rings 

 gradually decreasing in diameter, and each set tinted 

 with the pure prismatic hue which corresponds to its 

 size, from the extreme red to the extreme violet, in their 

 proper degrees of intensity ; he would produce just 

 such a series. If the diameters for all colours were 

 alike, the compound rings w^ould evidently be white and 

 infinite in number, separated by black intervals. If they 

 differed only a little starting from a common origin 

 the first ring would be nearly white, but exhibiting a 

 bluish border inwards and a reddish outwards, growing 

 more and more " pronounced," and broken into inter- 

 mediate tints in those beyond ; but if considerable, the 

 rincfs of different orders for different colours would soon 



mingle wuth and confuse each other's tints, creating the 

 sensation of uniform whiteness : thus accounting for the 

 comparative paucity of the mixed series. 



