indeed, all this statement may be abridged to a narrower form; a labour which, should we ourselves not be able to accomplish it, we bequeath to the active interest of contemporaries and posterity.
359.
With respect to the order of the chapters, it should be remembered that natural phenomena, which are even allied to each other, are not connected in any particular sequence or constant series; their efficient causes act in a narrow circle, so that it is in some sort indifferent what phenomenon is first or last considered; the main point is, that all should be as far as possible present to us, in order that we may embrace them at last from one point of view, partly according to their nature, partly according to generally received methods.
360.
Yet, in the present particular instance, it may be asserted that the dioptrical colours are justly placed at the head of the physical colours; not only on account of their striking splendour and their importance in other respects, but because, in tracing these to their source, much was necessarily entered into which will assist our subsequent enquiries.
361.
For, hitherto, light has been considered as a kind of abstract principle, existing and acting independently; to a certain extent self-modified, and on the slightest cause, producing colours out of itself. To divert the votaries of physical science from this mode of viewing the subject; to make them attentive to the fact, that in prismatic and other appearances we have not to do with light as an uncircumscribed and modifying principle, but as circumscribed and modified; that we have to do with a luminous image; with images or circumscribed objects generally, whether light or dark: this was the purpose we had in view, and such is the problem to be solved.