XXX.
TRANSITION.
357.
In looking back on the description and analysis of dioptrical colours, we do not repent either that we have treated them so circumstantially, or that we have taken them into consideration before the other physical colours, out of the order we ourselves laid down. Yet, before we quit this branch of our inquiry, it may be as well to state the reasons that have weighed with us.
358.
If some apology is necessary for having treated the theory of the dioptrical colours, particularly those of the second class, so diffusely, we should observe, that the exposition of any branch of knowledge is to be considered partly with reference to the intrinsic importance of the subject, and partly with reference to the particular necessities of the time in which the inquiry is undertaken. In our own case we were forced to keep both these considerations constantly in view. In the first place we had to state a mass of experiments with our consequent convictions; next, it was our especial aim to exhibit certain phenomena (known, it is true, but misunderstood, and above all, exhibited in false connection,) in that natural and progressive development which is strictly and truly conformable to observation; in order that hereafter, in our polemical or historical investigations, we might be enabled to bring a complete preparatory analysis to bear on, and elucidate, our general view. The details we have entered into were on this account unavoidable; they may be considered as a reluctant consequence of the occasion. Hereafter, when philosophers will look upon a simple principle as simple, a combined effect as combined; when they will acknowledge the first elementary, and the second complicated states, for what they are; then,