CHAPTER X.
ON THE CONSTRUCTION AND USE OFTHE TELESCOPIC KALEIDOSCOPE, FORVIEWING OBJECTS AT A DISTANCE.
We have already seen, in explaining the principles of the Kaleidoscope, that a symmetrical picture cannot be formed from objects placed at any distance from the instrument. If we take the simple Kaleidoscope, and holding an object-box in contact with the reflectors, gradually withdraw it to a distance, the picture, which is at first perfect in every part, will, at the distance of one-tenth of an inch, begin to be distorted at the centre, from the disunion of the reflected images; the distortion will gradually extend itself to the circumference, and at the distance of eighteen inches, or less, from the reflectors, all the symmetry and beauty of the pattern will disappear. An inexperienced eye may still admire the circular arrangement of the imperfect and dissimilar images; but no person acquainted with the instrument could endure the defects of the picture, even when the slightest distortion only is visible at the centre.
As the power of the Kaleidoscope, therefore, in its simple form, is limited to transparent objects, or to the outline of opaque objects held close to the aperture of the reflectors, it becomes a matter of consequence to extend its power by enabling it to produce perfectlysymmetrical patterns from opaque objects, from movable or immovable objects at a distance, or from objects of such a magnitude that they cannot be introduced before the opening of the reflectors. Without such an extension of its power, the Kaleidoscope might only be regarded as an instrument of amusement; but when it is made to embrace objects of all magnitudes, and at all distances, it takes its place as a general philosophical instrument, and becomes of the greatest use in the fine, as well as the useful arts.
Fig. 27.
Fig. 27.
In considering how this change might be effected, it occurred to me, that ifM N,Fig. 27, were a distant object, either opaque or transparent, it might be introduced into the picture by placing a lensL L, single or achromatic, at such a distance before the apertureA O B, that the image of the object may be distinctly formed in the air, or upon a plate of glass, the inner side of which was finely ground, and in contact with the endsA O,B Oof the reflectors, the plane passing throughA O B. By submitting this idea to experiment, I found it to answer my most sanguine expectations. The image formed by the lens atA O Bbecame a new object, as it were, and was multiplied and arranged by successive reflexions in the very same manner as if the objectM Nhad been reduced in the ratio ofM LtoL A, and placed close to the aperture.
Fig. 28.
Fig. 28.
The Compound or Telescopic Kaleidoscope is therefore fitted up as shown inFig. 28, with two tubes,A B,C D. The inner tube,A B, contains the reflectors as inFig. 27, and at the extremityC, of the outer tubeC D, is placed a lens which, along with the tube, may be taken off or put on at pleasure. The focal length of this lens should always be much less than the length of the outer tubeC D, and should in general be such that it is capable of forming an image at the end of the reflectors, whenA Bis pulled out as much as possible, and when the object is within three or four inches of the lens. When it is required to introduce into the picture very large objects placed near the lens, another lens of a less focal length should be used; and when the objects are distant, and not very large, a lens, whose principal focal length is nearly equal to the greatest distance of the lens from the reflectors, should be employed.
When this compound Kaleidoscope is used as a simple instrument for viewing objects held close to the aperture, the tubeA Bispushed in as far as it will go, the cell with the object-plate is slipped upon the endCof the outer tube, and the instrument is used in the same way as the simple Kaleidoscope.
In applying the compound Kaleidoscope to distant objects, the cell isremoved and the lens substituted in its place. The instrument is then directed to the objects, and the tubeA Bdrawn out till the inverted images of the objects are seen perfectly distinct, or in focus, and the pattern consequently perfectly symmetrical. When this is done, the pattern is varied, both by turning the instrument round its axis, and by moving it in any direction over the object to which it is pointed.
When the object is about four inches from the lens, the tube requires to be pulled out as far as possible, and for greater distances it must be pushed in. The points suited to different distances can easily be determined by experiment, and marked on the inner tube, if it should be found convenient. In most of the instruments there is, near the middle of the tubeA B, a mark which is nearly suited to all distances beyondthree feet. The object-plates held in the hand, or the mirror-box placed upon a table, at a distance greater than five or six inches, may be also used when the lensLis in the tube. The furniture of a room, books and papers lying on a table, pictures on the wall, a blazing fire, the moving branches and foliage of trees and shrubs, bunches of flowers, horses and cattle in a park, carriages in motion, the currents of a river, waterfalls, moving insects, the sun shining through clouds or trees, and, in short, every object in nature may be introduced by the aid of the lens into the figures created by the instrument.
The patterns which are thus presented to the eye are essentially different from those exhibited by the simple Kaleidoscope. Here the objects are independent of the observer, and all their movements are represented with the most singular effect in the symmetrical picture, which is as much superior to what is given by the simple instrument, asthe sight of living or moving objects is superior to an imperfect portrait of them. When the flame of a blazing fire is the object, the Kaleidoscope creates from it the most magical fireworks, in which the currents of flame which compose the picture can be turned into every possible direction.
In order to mark with accuracy the points on the tubeA B, suited to different distances, the instrument should be directed to a straight line, inclined likem n,Fig. 3, to the line bisecting the angular apertureA O B, and brought nearer to the centreOof the field. The perfect junction of the reflected images of the line at the pointsmʹ nʹ, &c., so as to form a star, or a polygon with salient and re-entering angles, will indicate with great nicety, that the tube has been pulled out the proper length for the given distance. In this way, a scale for different distances, and scales for different lenses, may be marked on the tube.
In the construction of the Tele-Kaleidoscope, as it may be called, the greatest care must be taken to have the lens of sufficient magnitude. If it is too small, the field of view will not coincide with the circular pattern, that is, the centre of the circular pattern will not coincide with the centre of the field; and this eccentricity will increase as the distance of the lens from the reflectors is increased, or as the object introduced into the picture approaches to the instrument. The boundary of the luminous field is also an irregular outline, consisting of disunited curves. These irregularities are easily explained. When the lens is too small, the luminous field is bounded by the brass rim in which the glass is fixed; and as this brass rim is at a distance from the reflectors, the portion of it presented tothe angular aperture cannot be formed by successive reflexions into a continuous curve; and for the same reason, the upper sectors of the luminous field are larger than the lower ones, and consequently the centre of the pattern cannot coincide with that of the field. In order to avoid these defects, therefore, the diameter of the lens should be such, that when it is at its greatest distance from the reflectors, the field of view may be bounded by the archA B,Fig. 13, and not by the brass rim which holds the lens. This may be readily known by removing the eye-glass, and applying the eye atEwhen the lens is at its greatest distance. If the eye cannot see the brass rim, then the lens is sufficiently large; but if the brass rim is visible, the lens is too small, and must be enlarged till it ceases to become visible. Sometimes the lens has been made so small that the brass rim is seen not only atA B, but appears also above the angular pointO, and produces a dark spot in the centre of the picture.
Instead of using two tubes, a lens is sometimes fitted into a tube about an inch longer than the focal length of the glass, and this tube is slipped upon the object endA B O,Fig. 21. This mode of applying the lens is, however, inferior to the first method, as there is little room for adjusting it to different distances; whereas with the long tube all objects at a greater distance than four inches from the lens may be introduced into the picture—a property which possesses very peculiar advantages.
The extension of the instrument to distant objects is not the only advantage which is derived from the use of the lens. As the position for giving perfect symmetry is rather within the extremities of thereflectors than without them; and as it is impossible to place movable objects within the reflectors, we are compelled to admit a small error, arising principally from the thickness of the objects, and from the thickness of the plate of glass which is necessarily interposed between the objects and the reflectors. The compound Kaleidoscope, however, is entirely free from this defect. The image of a distant, or even of a near object, can be formed within the reflectors, and in the mathematical position of symmetry; while, at the same time, the substitution of the image for the object itself, enables us to produce all the changes in the picture which the motion of the object could have effected, merely by turning the instrument round its axis, or by moving it horizontally, or in any other direction across the object. This instrument may be advantageously placed upon a stand like a telescope, and may either have a partial motion of rotation by means of a ball and socket, as shown in the figure, or what is better, a complete motion of rotation round the axis of the tubeC D, within a brass ring, occupying the place of the ball and socket.[5]