A chapter on entoptic colours, contained in the supplement to Goethe's works, was translated with the intention of inserting it among the notes, but on the whole it was thought most advisable to omit it. Like many other parts of the "Doctrine of Colours" it might have served as a specimen of what may be achieved by accurate observation unassisted by a mathematical foundation. The whole theory of the polarization of light has, however, been so fully investigated since Goethe's time, that the chapter in question would probably have been found to contain very little to interest scientific readers, for whom it seems chiefly to have been intended. One observation occurs in it which indeed has more reference to the arts; in order to make this intelligible, the leading experiment must be first described, and for this purpose the following extracts may serve. 3.[1] "The experiment, in its simplest form, is to be made as follows:--let a tolerably thick piece of plate-glass be cut into several squares of an inch and a half; let these be heated to a red heat and then suddenly cooled. The squares of glass which do not split in this operation are now fit to produce the entoptic colours. 4. "In our mode of exhibiting the phenomenon, the observer is, above all, to betake himself, with his apparatus to the open air. All dark rooms, all small apertures (foramina exigua),[2] are again to be given up. A pure, cloudless sky is the source whence we are derive a satisfactory insight into the appearances. 5. "The atmosphere being clear, let the observer lay the squares above described on a black surface, so placing them that two sides may be parallel with the plane of vision. When the sun is low, let him hold the squares so as to reflect to the eye that portion of the sky opposite to the sun, and he will then perceive four dark points in the four corners of a light space. If, after this, he turn towards the quarters of the sky at right angles with that where his first observation was made, he will see four bright points on a dark ground: between the two regions the figures appear to fluctuate. 6.