1813.] On the Measurement of Minute Particles. \t)$ 



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 carthamus 

 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, like those of the precipi- 

 tated 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, exhibit 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. The particles of the blood do 

 not derive their colour from any of the causes which have been 

 mentioned, since it may be extracted from them in a clear so- 

 lution. 



When I attempted to explain the colours of mixed plates, 

 which I had produced by partially moistening two lenses very 

 slightly convex, I observed that the reflection 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 bv 

 interference, as is naturally to be supposed on the principles of 

 the modulatory theory. But when the obliquity is so consider- 

 able, it is not very easy to assign a reason ibr this inversion; 

 and the experiments, which I have now mentioned, make it ne- 

 cessary to assume a law, which I cannot explain, that every very 

 oblique reflection inverts the properties of light with respect t'o 

 interference. This conclusion confirms the assertion of New- 

 ton, that a dark space, bordered by light, will appear in the cen- 

 tre ol a portion of light transmitted between the edges of two 

 knives placed very near each other, and the opinion of Mr. Jor- 

 dan, that the augmentation of a shadow by diffraction is to be 

 considered as the first dark space belonging to the coloured 

 binges. I had obtained a different result in an experiment si- 

 milar to Newton's, because I was not aware of the necessity of 

 OXiploying very sharp edges; for when the edges are blunt 'the 

 light la reflected from the one to the other in such a manner, as 

 Wholly to destroy the appearance of a central dark space; but in 

 any case this source of error may be avoided, by causing one of 

 the edges to advance a very little before the plane of the other 

 thai hall ol the fringes may disappear. It is, however, ne- 

 . to suppose this inversion confined to eases of extremely 

 oblique reflection, for when the deviation of the light from a 

 rectilinear path becomes a little more considerable, itoeffects are 

 .-.;-. , perceptible; the second and third fringes scarcely ever 

 requiring am material corrections of the calculations from which 

 (eluded. I he same inversiuii must also be attributed to the 

 Vol. II. N° 111. N 



