ON COLOURS OF THIN PELLICLES. 133 



sorts of colours, and those of the particles of bodie* sub- 'lies that ab- 

 jected to the laws oi" absorption. ■^^'^ ^* 



In the first place with respectto the latter the luminous The pencil not 

 pencil is not divided as in the former. The rays that do not ^'^^'^•'^ ^^'■''• 

 reappear in a given direction are not thf-own into another di- 

 rection ; they remain absorbed in the substance, even when 

 the mass is perftctly transparent. 



In the next place the colours resulting from absorption The colours 



are sometimes owins: to grrouus of rays very different from differ ^rom 



„? , ^ ',. . , T- . . , tliose of pelU- 



those that tnin pellicles can lurinsh. Tor mstance, these (,ies. 



never produce a compou.id colour like that of bodies tinged 



violet by oxide of manganese, or like the blue of cobalt or 



of indigo. Besides in these two kinds of phenomena there 



is no relation between the progress of colour depending on 



the degree of thickness. 



Thirdly the colours of the thinnest pellicles are very vivid. Are impercep- 

 Those of the most intensely coloured solutions on the con- ^^^^"^^ thia.^* 

 trary are imperceptible when so thin. It is for this reason 

 the colour of extremely thin leaves of mica has no relation to 

 the yeliow or other colour of the ma^s from which they are 

 separated ; they resembling pieces of the m^ost colourless 

 glass of similar thinness, so tliat mixed together they would 

 not be distinguishable. 



Thus glass, mica, or any other substance, which when Substances co- 

 very thin is invested with the most brilliant colours, pa-ses ' -"^^^ Y^'^? 

 to a colourless state by lacreasmg its thickness, or to a co- waen thicker. 

 lour iadependant of that displayed by it w hen thin. 



But it may be said, to compare a coloured mass to an Molecules of a 



assemblage of parcels of a determinate thickness, these g'^en thick- 



. , IT ness, kepr at a 



parcels must be kept at a suitable distance from each other, given di tance. 



In this case, I should answer, you will have a certain co- yf^,old reflect 

 lour reflected, aad another transmitted, which is precisely ou^i roiour, 

 complementary to the former. Now this double colouration '^^ opposite. 

 never takes place in perfectly diaphanous substances. 



The examples of the infusion of nephritic wood and pre- infuson of 



cipitates of gold are not more applicable to this case, since, nephn?,c wood 



11 1 ^1 n . n \ • • 1 & preci; itates 



as 1 iiave shown, the redected colours are owing to particles of gold, u .ake 



impermeable to hght, and disseminated in a transparent panicLs in a 



fluid ; and we may alter the nature of these particles, or even liuidf ^"^^^ 



have 



