Such are some of the interesting facts among those discovered by Mr. Thompson; and though we do not intend to push the inquiry further, we think it may not be without interest to describe here another illusion of that organ whose properties are in every way so curious and remarkable. [Illustration: Fig. 138.—Experiment on complementary colours.] [Illustration: Fig. 139.—Design for experiment or the _punctum cæcum_.] Another experiment to show the existence of impressions received by the retina can be made with the figure above (138). If the gaze be fixed upon the dark spot in the centre of the white figure for about half a minute, and the eyes then directed to the ceiling, or a sheet of white paper, the white figure will be reproduced _in black_. This result is based upon the principle of complementary colours. A red design, for instance, will be reproduced in green. There is a dark spot in every human eye—that is, a spot which is insensible to light. The eye is generally regarded as a perfect instrument, but it is not yet so by any means. One of our great philosophers remarked that if an instrument were sent home to him so full of errors he would feel justified in returning it to the optician. But the eye has its dark place, the _punctum cæcum_, and it can be discovered by covering the left eye with the hand, and holding the present page at arm’s length with the other. Then fix the gaze on the small cross in the picture, and bring the book close up. At a little distance the white ball will disappear from the page (fig. 139). [Illustration: Fig 140.—An optical illusion.] The illustration (fig. 140) shows us a very curious optical illusion, and one very easy to practise. Roll up a sheet of paper, and look through it, as through a telescope, with the right eye, _keeping both eyes open_. Then place the left hand open palm towards you against the roll of paper, you will then appear to be looking through a hole in your left hand. Sometimes the effect is produced without holding up the other hand to the roll, as shown in fig. 140. Among optical illusions there are a great number that may be produced by means of mirrors. The divided telescope is an example. The apparatus, raised on a firm stand, allows of one apparently seeing an object through a stone or other opaque object, as shown in fig. 141. The illustration shows the arrangement of the apparatus. The observer, looking through it, plainly perceives the object through the glass; the image is reflected four times before reaching his eye, by means of small mirrors concealed in the instrument. [Illustration: Fig. 141.—A divided telescope.]