OPTICAL ILLUSIONS CONTINUED—EXPERIMENTS—THE TALKING HEAD—GHOST ILLUSIONS.

The enumeration of optical illusions is so considerable that we have no intention of describing them all, and will merely cite a few other examples. The following facts have been communicated to us by M. Nachet:—

When examining algÆ under the microscope, we notice the spaces which separate the streaks ornamenting the silicious covering of these various organisms, and it is explained that they are formed by hexagons visible only when we examine the object with a powerful microscope. “For a long time,” says M. Nachet, “I occupied myself with the examination of the hexagonal appearance of the points constituting the streaks. Why should these hexagons show themselves, and how could they be other than the visible base of small pyramids piled very closely one on the other; and if this were the case, why were not the points of the little pyramids visible? Or, was the structure before me analogous to that of the eyes of insects? Then the carapace would be but a surface of perforated polygonal openings. This latter hypothesis was attractive enough, and would have explained many things; but some careful observations with very powerful object-glasses, quite free from blemishes, had shown me that these hexagons had round points, contrary to the descriptions of micrographs. These observations, corroborated by the micrographic photographs of Lackerbauer, the much-regretted designer, and by Colonels Woodward and Washington, left not the slightest doubt that it was necessary to discover why the eye persistently saw hexagons where there were circles. To elucidate this point, it was necessary to find some means of reproducing artificially what nature had accomplished with so much precision on the surfaces of algÆ. After many fruitless attempts, I decided on making a trial of a stereotype plate covered with dots arranged in quincunxes, very close together” (figs. 132 and 133). “The result was more successful than I had hoped; the effect produced is exactly that of the arrangement of the so-called hexagons of the most beautiful of the algÆ, the Pleurosigma angulata. If these stereotypes are examined with one eye only, we shall be immediately convinced that we have to do with hexagonal polygons.” It is useless to give any long exposition of a figure so clearly explanatory; it is simply an effect of the contrast and opposition of the black and white in the sensation of the retina. This effect is particularly striking with fig. 134, a negative photograph heliographically engraved according to fig. 133. In this the white points seem to destroy the black spaces, and to approach each other tangentially, and the irradiation is so intense that the white circles appear much larger than the black of fig. 133, although of the same diameter. There are in these facts many points which may interest not only students of micrography, but also artists. As to the algÆ, the origin of this investigation, it remains to be discovered if these circles which cover their silicious carapace are the projection of small hemispheres, or the section of openings made in the thick covering. Certain experiments, however, seem to prove that they are hemispheres, and the theory is also confirmed by a microscopic photograph from Lackerbauer’s collection, magnified 3,000 diameters, in which a black central point is seen in the centre of each circle, a certain reflection of the luminous source reproduced in the focus of each of the small demi-spheres which constitute the ornament of the algÆ. The microscope, which has progressively shown first the streaks, then the hexagons, and then the round points, will surely clear up the point some day or other.

Mr. Silvanus P. Thompson, Professor of Physics at University College, Bristol, has recently presented the French Society of Physical Science with a curious example of optical illusion, the true cause of which is not clearly known, but which we may compare with other facts made known some time ago, of which no precise explanation has been given. Let us first consider in what the effect discovered by Mr. S. P. Thompson consists, according to the description that has been given of it by M. C. M. Gariel; the illustrations here given will also allow of our verifying the truth of the statements.

The first illustration consists of a series of concentric circles of about the width of a millimetre, separated by white intervals of the same size (fig. 135). These dimensions are not absolute; they vary with the distance, and may even be a few inches in width if it is desired to show the phenomenon to a rather numerous auditory. If we hold the design in the hand, and give it a twirl by a little movement of the wrist, the circle appears to turn round its centre, and the rotation is in the same direction, and is equally swift; that is to say, the circle appears to accomplish a complete turn, whilst the cardboard really accomplishes one in the same direction. For the second effect we draw a dark circle, in the interior of which are placed a number of indentations at regular intervals (fig. 136). Operating in the same manner as described above, this notched wheel appears to turn round its centre, but this time in a different direction from the real movement. In this, however, as in the other design, the effect is more satisfactory if we do not look directly at it; the movements also are particularly striking in combinations such as that represented in fig. 137, in which the multiplicity of circles does not allow us to fix one specially. We may add that the same effects may be obtained with eccentric wheels, or even with other curves than circles. By means of a photograph on glass, Mr. Thompson has been able to reflect these designs on a screen where they were obtained on a large scale; a circular movement was communicated to the photographic plate, so that the design moved in a circular manner on the screen, and in this case also there existed the illusion that every circle seemed turning round its centre. And what is the explanation of these curious effects? Mr. Thompson does not believe (and we share his opinion) that the faculty possessed by the retina of preserving images during a certain time (persistence of impressions on the retina) can entirely explain these phenomena. Without desiring to formulate a decided theory, Mr. Thompson is of opinion that we may class these facts with others which have been known for some time, and that perhaps it is necessary to attribute to the eye some new faculty which may explain the whole at once.

Brewster and Adams have described phenomena which are equally curious, the principal of which we will describe, adding also some analogous investigations due to Mr. Thompson. The result seems to be that there exists in the eye a badly-defined purpose of nature, which in a certain way compensates (Brewster) for the real phenomenon, because it has a contrary effect, which will continue for some time after the cessation of the phenomena, and which gives by itself a sensation contrary to that which the real movement would have produced. Thus, after having fixed our eyes for two or three minutes on a rushing waterfall, if we suddenly turn our glance on the adjacent rocks, the latter appear to move from top to bottom. It is not a question here of the effect of the relative movement to be observed on regarding simultaneously the falling water and the rocks; if one can succeed in abstracting oneself to such an extent that the water appears motionless, the rocks appear to take a contrary movement. In the phenomenon we describe there is no simultaneous comparison; the eyes are turned successively first on the water, and then on the rocks. In a rapid river, such as the Rhine above the fall at Schaffhausen, the stream is not equally swift in every part, and the current is noticeably more rapid in the middle of the river than near the banks. If we look fixedly at the centre of the stream, and then suddenly turn our eyes towards the banks, it will appear as though the river were flowing back towards its source. This kind of compensation does not only produce an apparent displacement, but also changes in size. When travelling at great speed in a railway train, the objects of the surrounding country as one flies by them gradually appear smaller and smaller. If, when this occurs, we suddenly remove our eyes to the interior of the railway carriage, and fix them on immovable objects, such as the sides of the compartment, or the faces of our travelling companions, the images on the retina will really preserve the same size, and yet the objects will appear larger. 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.

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).

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.

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 utilized to facilitate the study of drawing. A piece of glass is fixed vertically on a black board (fig. 146). A model to copy from is placed on one side of the piece of glass, and is arranged so that the visual ray passes obliquely through the glass, and we perceive the reflection of the design very clearly on the other side. It is then very easily reproduced with a pencil on a sheet of white paper by tracing the outlines.