132 Action of Crystallized Bodies on Homogeneous Light. [Feb, 



nary pencils of* the same order for these two extremes, will ensure 

 that of the intermediate ones, at least very nearly. It would do 

 80 precisely, were the value of S a for any intermediate ray, such 

 a function of k as would result from making 6 constant in the 

 preceding equation, because the two laws, that of the dispersion 

 of the axes, and that of the magnitude of the rings of different 

 colours, would then act in exact opposition to each other 

 throughout their whole extent. It is in fact a case precisely 

 analogous to that of the compound achromatic prism, where, if 

 the law of dispersion in the one medium were identical with that 

 in the other, a perfectly colourless pencil would emerge, and 

 when these laws differ, the coincidence of the red and violet , 

 rays ensures an approximate coincidence of all the rest. Should 

 these laws, however, differ very considerably, an uncorrected • 

 colour will appear at the point so determined, and a nearer 

 approximation will be obtained by uniting two of the more 

 powerful intermediate rays, such as, for instance, the mean red 

 and the blue, or limit of the green and blue. 



This then is the origin of the virtual poles or points beyond 

 or between the axes where the tint rises to a white of the first 

 order, more or less feeble, or even to an absolute black ; and 

 we may now see the reason why the tints, in reckoning from 

 these points, approximate in a general way to the Newtonian 

 scale. In fact, the periods of the more refrangible rays being- 

 performed more rapidly than those of the less, if we suppose the * 

 coincidence above spoken of to take place at any point (the 

 minimum for instance) of the wth ring, the intervals between 

 the wth and {n + l)th minimum will be greatest for the red, and 

 least for the violet, &c. Consequently, when the violet next 

 disappears totally from the extraordinary pencil, there will 

 remain yet a little of the red, less of the orange, and so on, and 

 this difference increasing at every succeeding minimum on either 

 side, will produce a succession of colours approximating in a 

 general way to Newton's scale. This approximation will, how- 

 ever, be much less close on the side of the virtual pole towards 

 the nearest axis, because the disturbing influence of the separa- 

 tion of the axes on the figure of the rings and the law of their 

 successive intervals, is much more sensible than at a distance 

 from the pole. This will be evident if we consider that in the 

 interval between the extreme coloured axes, the tints will be 

 regulated entirely by the law of their distribution. Now this is. 

 perfectly corroborated by the succession of tints in the foregoing 

 tables, as well as by numerous experiments made on other 

 bodies. 



{To be cotitlnued,) 



