On changeable Colours and Glories. 298 
-pected. 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 lyco- 
perdon ; the green becoming somewhat yellower, when the 
gum, with which the colouring particles are mixed, fs di- 
Juted with water. But this is not the universal cause of a 
difference of colours exhibited by pigments in different 
Jights; the carthamus, or pink dye commonly sold for 
domestic use, affords an unequivocal instance of a sub- 
-stance 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 undeni- 
ably 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 glry, gives the colour op- 
posite 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 
yellow only; an infusiow, not too strong, gives the same 
colour, verging to orange, by direct transmitted light, and a 
bright blue by light reflected, or obliquely dispersed within 
the infusion, or at its surface. The solution of the car- 
thamus affords no green reflection from its surface, and 
varies in its hue, in different lights, only from crimson to 
scarlet. The tinging particles of the lignum nephriticum, 
dike those of the precipitated lead and silver, are probably 
extremely minute, since the colour is but little changed by 
changing the density of the fluid. It often happens that a 
blue colour, precisely like that of this infusion, is reflected 
by green glass bottles, which, when seen by transmitted 
light, exbibit only a reddish brown colour. The inner bark 
of the ash is also said to have a property similar to that of 
the lignum nephriticum. (Murr. App. Med.) The particles 
of the blood do not derive their colour from any of the 
causes which have been mentioned, since it may be ex- 
tracted from them in a clear solution. . 
‘¢ When I attempted to explain the colours of mixed 
pistes, which I had produced by partially moistening two 
enses very slightly convex, I observed that the reflction 
of the light from the internal surface of a denser medium 
must be supposed to invert its properties with respect to the 
production of colours by interference, as is naturally to, be 
; ih Mek A ' supposed 
