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). 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

Then

through a telescope, with the right eye, keeping both eyes open. 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.

Convex or concave mirrors distort images in a singular manner, and produce very interesting effects. Anamorphoses constitute particular objects belonging specially to the class of experiments relating to cylindrical mirrors. They are images made according to determined rules, but so distorted that when regarding them fixedly we can only distinguish confused strokes. When they are seen reflected in the curved mirrors, they present, on the contrary, a perfectly regular appearance. Fig. 142 exhibits an Anamorphosis made by a cylindrical mirror. It will be seen that the confused image of

the horizontal paper is reflected in the cylindrical mirror, producing the figure of a juggler. It is easy to contrive similar designs one's-self; and comical mirrors may also be employed which produce particular effects of a no less interesting kind. The next illustration is of a set of figures which in a cylindrical mirror look like the ten of hearts (fig. 143).

One of the most remarkable applications of mirrors in amusing experiments is undoubtedly that of the severed and talking head. A few years back this trick obtained considerable success in Paris and a number of other towns. The spectators beheld a small space set apart, in which was placed a table on three legs; on this table was a human head, placed in cloth on a dish. The head moved its eyes and spoke; it evidently belonged to a man whose body was completely hidden. The spectators thought they saw an empty

space beneath the table, but the body of the individual who was really seated there was concealed by two glasses placed at an inclination of 45° to the

walls on the right and left. The whole was arranged in such a manner that the reflection of the walls coincided with the visible part of the wall at the

back of the room. The three walls were painted in one colour, and a subdued light increased the illusion, the effect of which was remarkable (fig. 144).

The spectres designed by Robin also attracted considerable public

attention within recent times. They were images formed by the medium of transparent glass. Glass panes often produce the phenomenon of spectres. In the evening, when it is dark out of doors, it is easy to prove that the reflection of objects in a lighted room can be reproduced behind the window panes by reason of the darkness outside. If we approach the window pane,

we see also the real objects outside, a balcony, tree, etc. These real objects mingle with the reflected image, and combine to produce very curious effects. In this way M. Robin has contrived the effects of the theatre. He throws on the stage the reflection of a person dressed as a Zouave, and he himself,

armed with a sabre, stabs the spectre through the body. A great number of other singular effects have been obtained in the same manner. Pepper's Ghost was managed in this way.

Within recent times, images produced in a similar way have been