1 92 On the Measurement of Minute Particles. [Sept . 



water, I had the pleasure of finding my conjecture confirmed ; 

 for when the water was saturated with salt, the yellow green 

 became nearly blue, and the purple redder or browner ; and 

 when olive oil was employed, the light directly transmitted 

 was purple, and the oblique light greenish ; in balsam of Tolu 

 again, this purple became red, and the indirect light afforded a 

 faint blue. In air too, I found that the powder appeared of a 

 bright blue green by direct light, and of a purplish hue with a 

 light a little oblique ; but when the obliquity became a little 

 greater, the tint changed to a brownish yellow green, which 

 continued afterwards unchanged ; this alteration may perhaps 

 be derived from the admixture of a portion of light coming 

 round the particles by a more circuitous route. By comparing 

 the opposite effects of water and olive oil, of the refractive den- 

 sities 1*336 and 1*379, the refractive density of the particles 

 themselves may be calculated to be 1 *62, or somewhat less. 



Grey beaver wool seems of a purplish hue in direct, and 

 greenish in oblique light, both in air and in olive oil ; its grey 

 colour seems to be derived from a mixture of these tints ; in 

 olive oil, the rings of colours which it affords are considerably 

 altered in their appearance, the reds becoming every where very 

 faint. Lead precipitated from its acetate, or silver from its 

 nitrate, by common water, affords a reddish direct and a bluish 

 indirect light, and the same seems to be true of smoke, and of 

 other bodies consisting of very minute particles: but when the 

 indirect light is very powerful, smoke sometimes appears reddish 

 in it, as might be expected from a collection of very small 

 opaque instead of transparent particles. 



Mr. Delaval has observed that an infusion of sap green appears 

 of a bright red by transmitted light, and the case seems perfectly 

 analogous to that of the dust of the lycoperdon : the green 

 becoming somewhat yellower, when the gum, with which the 

 colouring particles are mixed, is diluted with water. But this is 

 not the universal cause of a difference of colours exhibited by 

 pigments in different lights; the carthamus, or pink dye com- 

 monly sold for domestic use, affords an unequivocal instance of 

 a substance exhibiting colours analogous to those of thin plates, 

 which have been adduced by Newton, in illustration of the 

 colours of natural bodies; the reflected light being undeniably of 

 a yellow green, while the transmitted light is of a bright pink 

 colour. Here the light regularly reflected from the surface only, 

 especially when dry, gives the colour opposite to that of the 

 transmitted light ; all the light passing through the fluid, even 

 indirectly, giving a pink colour. But the infusion of the lignum 

 nephriticum seems to hold a middle place between this substance 

 and those which have been mentioned before; the dry extract is 

 of a brownish yellcw only; an infusion, not too strong, gives 



