Z0'ETROPE, or WHEEL OF LIFE, an optical instrument, so named from its exhibiting pictures of objects as if endowed with life and activity. Although only of late years introduced from America, under this name, the instrument itself, which is simply a cylindrical thaurnatrope, was invented and made the subject of a patent so long ago as the year 1860. Mr. Peter Hubert the inventor, in his published specification, has very fully described various modifications of the cylindrical thaumatrope ; and the instruments which, under the name of mirnoscope, he contributed to the London exhibi tion of 1S62, being furnished with pictures of exquisite artistic finish and beauty, were deservedly rewarded with " honorable mention." Mr. Desvignes also proposed a stere oscopic form of the instrument, in anticipation of tho•one already described under stere oscope (q.v.), and has employed models, insects, and other objects, instead of pictures, with perfect success. The truly marvelous results shown in this instrument depend, primarily, on the well-known fact that vision " persists" for a certain short interval of time after the occlusion of the visual ray. It follows from this principle that if a series of pictures, representing the different attitudes successively assumed by an object in completing a given movement, be presented to the eye so quickly that the visual impres sion of each picture shall continue until the incidence of the one next following, the object will remain constantly in view, and its various parts will appear to execute the movement delineated by the pictures. The zoetropc in its most popular but by no means most excellent form consists of a cylinder of strong card-board, 12 in. in diam eter, and 7/ in. in depth, with a metal rim at the top, and fastened to a circular piece of wood. The latter is screwed at its center to a pivot, which moves freely within an upright of a stand, and forms a vertical axis, round which the cylinder may be made to• revolve with any desired rapidity. There arc 13 equidistant and vertical apertures, each A- in. in width, and 3 in. long. Each series of pictures is printed on a strip of thick paper, 3i in. in breadth, and 30 in. in length. In using the instrument, illuminate it well from above, and, having placed the picture-strip within the cylinder, immediately beneath the apertures, rotate the cylinder with the requisite velocity (which will vary according to the nature of the subject), and look through the apertures at the pictures upon the opposite side of the cylinder. To avoid the grotesqueness of a number of groups all seen performing the same movement at the same time, Mr. Desvignes devised the thaumatrope, in which the cylinder turns upon a horizontal axis, and by means of an aperture at the bottom of a hood the sight is limited to the space occupied by a sin gle group on the opposite side of the cylinder, and, both eyes being brought into con sentaneous activity, the clearness and pleasantness of the illusion are greatly enhanced. In this arrangement, the groups are arranged, on the strips, one above another, and not side by side, as in the former; and they are perforated with apertures to correspond with those on the cylinder. As has been pointed out by Mr. Desvignes, the width of the apertures should never exceed one-sixteenth of an inch ; and the exterior of the instru ment should be painted mat black.
We -will now speak of the principles which determine the character and distinctness of the illusion, and first of all will endeavor to explain why it is that the piotures, looked at through the apertures, are plainly visible; whereas, viewed over the top of the revolving cylinder, they run into an indistinguishable mass.
The effect of the apertures, in this regard, is twofold, they limit the time during which each group is pictured upon the retina; and they limit, or should be made te limit, the quantity of each group at any one instant so depicted. Obviously, if the view be instantaneous only, the group in that minute interval of time will not have moved to a perceptible degree, and will therefore appear stationary. It is for this reason that the apertures should be made very narrow (not exceeding one-sixteenth of an inch, as above stated), and then, the eye being placed close to, them, the retinal images will not be slurred —as with wider apertures, and consequent increased motion of the pictures while under view, they would be—and the maximum of definition is thus attained. But, in this case, both the quantity of light reaching the eyd, and the time of its operation, are insuffi cient, which defects can only be compensated by setting the instrument in violent motion, thus occasioning an inconveniently rapid recurrence of the retinal impressions. Instead, therefore, of letting the whole picture be seen for a single instant, let us increase the total time of visibility by arranging that successive small portions of the pictures shall be seen in suceessire instants. This is effected by withdrawing the eye a few inches from the aperture; for the eye being at the apex of the visual angle, the further it is removed from the aperture, the greater is the distance within the visual angle through which the aperture must travel; and prolonged in a like degree will be the total time ef viability for each picture. But here another difficulty is evoked, manifesting itself in one of the most puzzling phenomena of this curious instrument. The pictures are seen as sensibly diminished in breadth, the explanation of which is as follows: The longer the time of visibility, the greater will be the onward movement of the picture while under view ; therefore, if the left-hand side of the picture come first into view, by the time the advance of the aperture permits of the right-hand side being seen, the latter will have progressed toward the left, and it will accordingly be seen relatively nearer to the left side than is its natural position. And this compression, of course, takes effect over the whole of the picture. In the exquisitely elaborate drawings prepared by Mr. Desvignes, this compression was allowed for, to the great enhancement of the effect.
Lastly, of the effects due to the pictures themselves. If the pictures on the strip be the same in number as the apertures; the pictures will either coincide with the aper tures, in point of position, or will be equidistant, each from its adjacent aperture: in which case there will be no apparent shifting of the pictures either to right or left. But if there be 12 pictures and 13 apertures, the picture-interval will be greater than the aperture-interval, and only one picture on the strip can be coincident with an aperture; the rest will be situated in advance of the apertures, each succeeding one slightly more than its predecessor; and, as a consequence, seen through the apertures of the revolving cylinder, they will appear continuously to advance. The contrary will bo the case, if the number of the pictures exceed that of the apertures. Thew curious properties, which make it possible to exhibit figures moving either backward or forward, as well as with movements proper to themselves, were fully investigated by the late prof. Faraday; and we would recommend those interested in the subject to refer to his memoir on optical deceptions, contained in his Experimental Researches in Chemistry and Physics, pp. 291, et seq.