This paper is intended to prove that the colour of blue in the sky and in the landscape is simply the result, in the former, of the darkness of space, as seen through the white light contained in the atmosphere; and, in the latter, of the same cause as shown in the dark and distant portions of the landscape being viewed through the interposed medium of air filled with white light. The colour of aërial blue being due in both cases to the same cause, namely—a dark body neutralised as to its darkness by being seen through a white and transparent medium.