order that the practical deductions we look to should be really useful, we should theorise without forgetting that we are so doing, we should theorise with mental self-possession, and, to use a bold word, with irony.
In the second part4 we examine the Newtonian theory; a theory which by its ascendancy and consideration has hitherto impeded a free inquiry into the phenomena of colours. We combat that hypothesis, for although it is no longer found available, it still retains a traditional authority in the world. Its real relations to its subject will require to be plainly pointed out; the old errors must be cleared away, if the theory of colours is not still to remain in the rear of so many other better investigated departments of natural science. Since, however, this second part of our work may appear somewhat dry as regards its matter, and perhaps too vehement and excited in its manner, we may here be permitted to introduce a sort of allegory in a lighter style, as a prelude to that graver portion, and as some excuse for the earnestness alluded to.
We compare the Newtonian theory of colours to an old castle, which was at first constructed by its architect with youthful precipitation; it was, however, gradually enlarged and equipped by him according to the exigencies of time and circumstances, and moreover was still further fortified and secured in consequence of feuds and hostile demonstrations.
The same system was pursued by his successors and heirs: their increased wants within, the harassing vigilance of their