368 LECTURE XXXIX. 



then to crimson and blue, after which the effect is destroyed by the opacity 

 which the oxid acquires. Usually, however, the series of colours produce d 

 in reflected light follows an order somewhat different : the scale of oxid 

 is denser than the air, and the iron below than the oxid ; but where the 

 mediums above and below the plate are either both rarer or both denser 

 than itself, the different natures of the reflections at its different surfaces 

 appear to produce a modification in the state of the undulations, and the 

 infinitely thin edge of the plate becomes black instead of white, one of the 

 portions of light at once destroying the other, instead of cooperating with 

 it. Thus when a film of soapy water is stretched over a wine glass, and 

 placed in a vertical position, its upper edge becomes extremely thin, and 

 appears nearly black, while the parts below are divided by horizontal lines 

 into a series of coloured bands ; and when two glasses, one of which is 

 slightly convex, are pressed together with some force, the plate of air 

 between them exhibits the appearance of coloured rings, beginning from 

 a black spot at the centre, and becoming narrower and narrower, as the 

 curved figure of the glass causes the thickness of the plate of air to increase 

 more and more rapidly. The black is succeeded by a violet, so faint as to 

 be scarcely perceptible ; next to this is an orange yellow, and then crim- 

 son and blue. When water or any other fluid, is substituted for the air 

 between the glasses, the rings appear where the thickness is as much less 

 than that of the plate of air, as the refractive density of the fluid is 

 greater ; a circumstance which necessarily follows from the proportion of 

 the velocities with which light must, upon the Huygenian hypothesis, be 

 supposed to move in different mediums. It is also a consequence equally 

 necessary in this theory, and equally inconsistent with all others, that 

 when the direction of the light is oblique, the effect of a thicker plate must 

 be the same as that of a thinner plate, when the light falls perpendicularly 

 upon it ; the difference of the paths described by the different portions of 

 light precisely corresponding with the observed phenomena. (Plate XXX. 

 Fig. 447... 449.) 



Sir Isaac Newton supposes the colours of natural bodies in general to be 

 similar to these colours of thin plates, and to be governed by the magni- 

 tude of their particles. If this opinion were universally true, we might 

 always separate the colours of natural bodies by refraction into a number of 

 different portions, with dark spaces intervening ; for every part of a thin 

 plate which exhibits the appearance of colour, affords such a divided 

 spectrum, when viewed through a prism. There are accordingly many 

 natural colours in which such a separation may be observed ; one of the 

 most remarkable of them is that of blue glass, probably coloured with 

 cobalt, which becomes divided into seven distinct portions. It seems, 

 however, impossible to suppose the production of natural colours perfectly 

 identical with those of thin plates, on account of the known minuteness of 

 the particles of colouring bodies, unless the refractive density of these par- 

 ticles be at least 20 or 30 times as great as that of glass or water ; which is 

 indeed not at all improbable with respect to the ultimate atoms of bodies, 

 but difficult to believe with respect to any of their arrangements consti- 

 tuting the diversities of material substances. 



