RAINBOWS. 329 



prismatic colours are seen on its surface when the thinness is of 

 sufficient extent ; in the same kind of way a drop of oil, or tar, or 

 other fluid lighter than water and insoluble therein, if placed on 

 the surface of a large mass of water, will gradually spread over the 

 surface, forming a very thin layer which will show prismatic colours. 

 This effect is produced in consequence of the reflection of light from 

 both the upper and under surfaces of the thin film, as in Expt. 343 

 with a glass plate, the two reflections causing an action termed 

 interference to take place between the two rays when the film 

 is of less than a certain thickness ; in consequence of which rays 

 of different refrangibility become more readily visible in one posi- 

 tion than at another, producing the sensation of patches of different 

 colours, somewhat like a spectrum received on a screen after a beam 

 of white light has passed through a prism (Expt. 349). The 

 " nacre " or peculiar coloured lustre of pearls and mother of pearl, 

 the tints of opals and many other analogous substances, and the 

 coloured films developed with melted lead or bismuth in Expts. 15 

 and 1 8 are all produced by actions of this kind ; the production of 

 rainbow colours in this fashion is generally termed iridescence. 

 The formation of rainbow-tinted haloes during certain kinds of misty 

 or foggy weather, and of coloured fringes round candles, &c., when 

 looked at through glass covered with very fine globules of con- 

 densed dew, or dusted over with fine lycopodium powder, &c., are 

 closely related phenomena, the full explanation of which must be 

 sought for in larger works. 



Expt. 360. The Rainbow. Whenever small drops of water, 

 such as rain or the spray of waterfalls, fountains, &c., are suspended 

 for the time being in the air, and a beam of white light (natural, 

 such as that from the sun and moon, or artificial, as the electric 

 light, &c.) is allowed to fall thereon, the light enters each 

 droplet and is refracted, the rays being partly reflected from 

 the posterior surf ace of 

 the drop and then again 

 refracted outwards, 

 ultimately forming an --^ 

 angle with their origi- 

 nal path of less than v 

 a right angle. Under ~~"" - 

 favourable conditions 

 a ray of light may 

 meet the eye of an 

 observer in two ways 

 by means of actions Fi S- l75 - Primary Rainbow, 



of this sort : firstly, when reflection takes place once in the drop ; 



