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.
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.
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.