448 ON THE ABSORPTION OF LIGHT BY COLOURED MEDIA, 



that the tint of the emergent pencil is essentially dependent 

 on the thickness of the medium, and that it is only from a 

 knowledge of the relative values of y, y, &c. in the various 

 parts of the spectrum, that we can say, a priori, whether the 

 tint of a thick glass will retain any similarity to that of a thin 

 one of the same kind. It is evident that, y being necessarily a 

 fraction less than unity, the values of y, y ', &c. will all diminish, 

 with an increase in that of t. Those terms, however, in which 

 y is smallest, will decrease most rapidly, and, however trifling 

 may be the difference in this respect, it is always possible to 

 render its effect sensible on the tint, by taking t sufficiently 

 large. Thus the water of the Lake of Geneva is indigo-blue, 

 that of the Lake of Como emerald- green, when viewed through 

 a considerable thickness, though colourless in small quantities. 

 If (as is very common) the value of y should have two une- 

 qual maxima, in different parts of the spectrum, and if, at the 

 same time, the greater of these should happen to corres- 

 pond to a ray of feebler illuminating power than the less, the 

 tint, in small thicknesses of the medium, will (generally speak- 

 ing) be that of the lesser maximum, the greater vividness of 

 these rays giving them a predominance over the others, though 

 more numerous ; but as this inequality of number increases 

 with the increase of thickness, the feebler rays will at length 

 begin to influence the tint, and finally obtain the predomi- 

 nance ; thus producing, in several cases, a complete change of 

 colour, not a little surprising to those who are ignorant of its 

 cause. Dr Thomson's muriated liquor (chloride of sulphur), 

 which is yellowish-green in very small thicknesses, and bright 

 red in considerable ones, is a case in point. A solution of 

 sap-green presents the same phenomenon yet more striking- 

 ly. If inclosed between glass-plates slightly inclined, so as to 

 form a thin wedge, its colour towards the edge will appear 



emerald- 



