636 



ACCOUNT OP SOME CASES 



It appears to be necessary for theproduc- 

 toon of these colours, that the glasses be held 

 nearly in a right line between the eye and 

 the common termination of a dark and lu- 

 minous object: the portion of the rings, seen 

 on the dark ground, is then more distinct 

 than the remaining portion ; and, instead of 

 being continuations of the rings, they exhi- 

 bit every where opposite colours, so as to re- 

 semble the colours of common thin plates 

 seen by reflection, and not by transmission. 



In order to understand this circumstance, 

 we must consider, that where a dark object 

 (as A, Plate 15. Fig. 1 1 2.) is placed behind 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 surface, (as D, E ;) while the 

 light, which passes through those parts of the 

 glasses which are on the side opposite to 

 the dark object, consists of rays refracted 

 as before through the edges, (as F, G,) 

 or simply passing through the fluid (as 

 H, J..) The respective combinations of 

 these portions- of light exhibit series of 

 colours in diflierent orders, since the inter- 

 nal reflection modifies the interference of the 

 jays 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 co- 

 lours 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. 



In applying the general law of interfer- 

 ence to these colours^ as well as to those of 

 thin plates already known, it is impossible to 

 avoid a supposition, which is a part of the 

 undulatory theory, that is, that the velocity 



of light is the greater, the rarer the medium ; 

 and there is also a condition annexed to this 

 explanation of the colours of mixed plates, as 

 as well as to that of the colours of simple thin 

 plates, wli ich involves another part of the same 

 theory; that is, that where one of the portions 

 of light has been reflected at the surface of a 

 rarer medium, it must be supposed to be re- 

 tarded one half of the appropriate interval ; 

 for instance, in the central black spot of a 

 soap bubble, where the actual lengths of the 

 paths very nearly coincide, but the effect is 

 the same as if one of the portions had been 

 so retarded as to destroy the other. From 

 considering the nature of this circumstance, 

 I ventured to predict, that if the two reflec- 

 tions were of the same kind, made at the 

 surfaces of a thin plate, of a density inter- 

 mediate between the densities of the me- 

 diums containing it, the effect would be re- 

 versed, and the central spot, instead of black, 

 would become white; and I have now the 

 pleasure of stating, that I have fully verified 

 this conclusion, by interposing a drop of oil 

 of sassafras between a prism of flint glass and 

 a lens of crown glass : the central spot seen 

 by reflected light was white, and surrounded 

 by a dark ring. It was however necessary to 

 use some force, in order to produce a con- 

 tact sufficiently intimate ; and the white 

 spot differed, even at last, in the same de- 

 gree from perfect whiteness, as the black 

 spot usually does from perfect blackness. 

 There are also some irregularities attending 

 the phenomena exhibited in this manner by 

 different refracting substances, especially 

 when the reflection is total, which deserve 

 further investigation. 



The colours of mixed plates suggested to 

 me an idea, which appears to lead to an ex- 



