of a window when they are relieved on a grey sky.28
217.
In all this we are never to forget that this appearance is not to be considered as a complete or final state, but always as a progressive, increasing, and, in many senses, controllable appearance. Thus we find that, by the negation of the above five conditions, it gradually decreases, and at last disappears altogether.
XV.
EXPLANATION OF THE FOREGOING PHENOMENA.
218.
Before we proceed further, it is incumbent on us to explain the first tolerably simple phenomenon, and to show its connexion with the principles first laid down, in order that the observer of nature may be enabled clearly to comprehend the more complicated appearances that follow.
219.
In the first place, it is necessary to remember that we have to