( 376 ) 



oil is to be considered^ and this is much smaller than the 

 diiTerence bctweeii air and water. 



It appears to be necessary for the production of these 

 colours, that the glasses be held nearly in a right line be- 

 tween the eye and the common termination of a dark and 

 luminous object; the portion of the rings, seen on the 

 dark ground, is then more distinct than the remaining por- 

 tion; and, instead of being continuations of the riugs, they 

 exhibit every where opposite colours, so as to resemble 

 the colours of common thin plates seen bj reflection, and 

 not bj transmission. 



In order to understand this circumstance, we must con- 

 sider, that where a dark object (as A fig. 1) is placed be- 

 hind the glasses, the whole of the light, which comes to 

 the eye, is either refracted through the edges of the drops, 

 (as the rays B,C,) or reflected from the internal sarface (as 

 D,E ;) while the light, which passes through those parts of 

 the glasses which are on the side opposite to the dark ob- 

 ject, consists of rays refracted as before through the edges, 

 (as r, G,) or simply passing through the fluid (as H, I.) 

 The respective combinations of these portions of light exhibit 

 series of colours in dilFerent orders, since the internal re- 

 flection modifies the interference of the rays on the side of 

 the dark object, in the same manner as in the common 

 colours of thin plates, seen by reflection. When no dark 

 object is near, both these series of colours are produced 

 at-once; and since they are always of an opposite nature 

 at any given thickness of a plate, they neutralise each 

 other, and constitute white light.^" 



:i. Na YOUNG heeft de Heer babinet in geschrifte over 

 dit onderwerp gehandeld in eenen brief aan arago ^) ; ook 



*) Cowptes Rendus, 1839, Vol. VIII. p. 306 et 307. 



