Popular Scientific Recreations in Natural Philosphy, Astronomy, Geology, Chemistry, etc., etc., etc. Gaston Tissandier

top. It is a metallic toy put in motion by means of a cord wound round a groove. The axis is hollow, admits a metallic stem, and fits into a handle which is held in the hand. The top is placed upon a little cup in an upright position, and it is then set spinning in the usual way with the cord. The stem and handle are then withdrawn, and as the top will continue to spin for a long time, discs and various outline shapes can be fixed upon it, and various objects will be shadowed thereon. Cups, bowls, candlesticks, and jugs can be seen plainly revolving as the top carries the wire representation in outline rapidly past the eyes. Coloured cardboard can be worked into various patterns, and much amusement will be created amongst children and young people.

      

CHAPTER XII.

       Table of Contents

      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:—

      Fig. 132.—Hexagonal appearance formed by circles joined together.

      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.

      Fig. 133.—Another figure of the same kind.

      Fig. 134.—Third figure.

      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.

      Fig. 135. and Fig. 136.

       Mr. Thompson’s optical illusion. Give a circular movement to these figures, and the circles will appear to turn round.

      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.

      Fig. 137.—Another figure of Mr. Thompson’s. The different circles appear to turn round if we give the design a rotating movement.

      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.