[2] The author has omitted the orange and purple in the coloured diagrams which illustrate these first experiments, from a wish probably to present the elementary contrast, on which he lays a stress, in greater simplicity. The reddish tinge would be apparent, as stated above, where the blue and yellow are in contact with the black.--T. [3] Plate 2, fig. 2 [4] Plate 2, fig. 4 [5] In this case, according to the author, the refracting medium being increased in mass, the appearance of colour is increased, and the displacement is greater.--T. [6] Plate 2, fig. 3. [7] Fig. 2, plate 1, contains a variety of forms, which, when viewed through a prism, are intended to illustrate the statement in this and the following paragraph. XIV. CONDITIONS UNDER WHICH THE APPEARANCE OF COLOUR INCREASES. 209. We have seen in the foregoing experiments that all appearance of colour occasioned by refraction depends on the condition that the boundary or edge be moved in upon the object itself, or the object itself over the ground, that the figure should be, as it were, carried over itself, or over the ground. And we shall now find that, by increased displacement of the object, the appearance of colour exhibits itself in a greater degree. This takes place in subjective experiments, to which, for the present, we confine ourselves, under the following conditions. 210. First, if, in looking through parallel mediums, the eye is directed more obliquely. Secondly, if the surfaces of the medium are no longer parallel, but form a more or less acute angle. Thirdly, owing to the increased proportion of the medium, whether parallel mediums be increased in size, or whether the angle be increased, provided it does not attain a right angle. Fourthly, owing to the distance of the eye armed with a refracting medium from the object to be displaced. Fifthly, owing to a chemical property that may be communicated to the glass, and which may be afterwards increased in effect. 211. The greatest change of place, short of considerable distortion of the object, is produced by means of prisms, and this is the reason why the appearance of colour can be exhibited most powerfully through glasses of this form. Yet we will not, in employing them, suffer ourselves to be dazzled by the splendid appearances they exhibit, but keep the above well-established, simple principles calmly in view. 212. The colour which is outside, or foremost, in the apparent change of an object by refraction, is always the broader, and we will henceforth call this a _border_: the colour that remains next the outline is the narrower, and this we will call an _edge_. 213. If we move a dark boundary towards a light surface, the yellow broader border is foremost, and the narrower yellow-red edge follows close to the outline. If we move a light boundary towards a dark surface, the broader violet border is foremost, and the narrower blue edge follows. 214.