and this hurried and rounded in writing, is at length 7; it being much easier to write a line inclined I, than a perpendicular line I.

ART. XVII. History of Dr. Brewster's Kaleidoscope, with Remarks on its supposed resemblance to other combinations of plain Mirrors.

As this instrument has excited great attention, both in this country and on the Continent, we have no doubt that our readers will take some interest in the history of the invention. In the year 1814, when Dr. Brewster was engaged in experiments on the polarisation of light by successive reflexions between plates of glass, which were published in the Philosophical Transactions for 1815, and honoured by the Royal Society of London with the Copley Medal, the reflectors were in some cases inclined to each other, and he had occasion to remark the circular arrangement of the images of a candle round a centre, or the multiplication of the sectors formed by the extremities of the glass plates. In repeating, at a subsequent period, the experiments of M. Biot on the action of fluids upon. light, Dr. B. placed the fluids in a trough formed by two plates of glass cemented together at an angle. The eye being necessarily placed at one end, some of the cement which had been pressed through between the plates, appeared to be arranged into a regular figure. The symmetry of this figure being very remarkable, Dr. B. set himself to investigate the cause of the phenomenon, and in doing this he discovered the leading principles of the kaleidoscope. He found that in order to produce perfectly beautiful and symmetrical forms, three conditions were necessary.

1. That the reflectors should be placed at an angle, which was an even or an odd aliquot part of a circle, when the object was regular; or the even aliquot part of a circle when the object was irregular.

6. That out of an infinite number of positions for the object both within and without the reflectors, there was only one position where perfect symmetry could be obtained, namely, by placing the object in contact with the ends of the reflectors.

3. That out of an infinite number of positions of the eye, there was only one where the symmetry was perfect, namely, as near as possible to the angular point, so that the circular field could be distinctly seen; and that this point was the only one out of an infinite number at which the uniformity of the light of the circular field was a maximum.

Upon these principles I r. B. constructed an instrument in which he fixed permanently across the ends of the reflectors, pieces of coloured glass, and other irregular objects, and he shewed the instrument in this state to some Members of the Royal Society of Edinburgh, who were much struck with the beauty of its effects. In this case, however, the forms were nearly permanent, and a slight variation was produced by varying the position of the instrument, with respect to the light. The great step however, towards the completion of the instrument, remained yet to be made, and it was not till some time afterwards, that the idea occurred to Dr. B. of giving motion to objects, such as pieces of coloured glass, &c. which were either fixed or placed loosely in a cell at the end of the instrument. When this idea was carried into execution, the kaleidoscope in its simple form was completed.

In this state, however, the kaleidoscope could not be considered as a general philosophical instrument of universal application; for it was incapable of producing beautiful forms, unless the object was nearly in perfect contact with the end of the reflectors.

The next, and by far the most important step of the inven tion, was therefore to remove this limitation by employing a draw tube and lens, by means of which beautiful forms could

be created from objects of all sizes, and at all distances from the observer. In this way the power of the kaleidoscope was indefinitely extended, and every object in nature could be introduced into the picture, in the same manner as if these objects had been reduced in size, and actually placed at the end of the reflectors.

When the instrument was brought to this state of perfection, Dr. Brewster was urged by his friends to secure the exclusive property of it by a patent, and he accordingly took out a patent for " a new optical instrument for creating and exhibiting beautiful forms." In the specification of his patent, he describes the kaleidoscope in two different forms. The first consists of two reflecting planes, put together according to the principles already described, and placed in a tube, with an eye-hole in the particular position which gives symmetry and a maximum uniformity of light, and with objects such as coloured glass, placed in the position of symmetry, and put in motion either by a rotatory movement, or by their own gravity, or by both combined. The second form of the instrument, described in the specification, is, when the tube containing the reflectors is placed in a second tube, at the end of which is a convex lens, which introduces into the picture objects of all magnitudes, and at every distance, as has been already described.

After the patent was signed, and the instruments in a state of forwardness, the gentleman who was employed to manufacture them under the patent, carried a kaleidoscope to shew to the principal London opticians, for the purpose of taking orders from them. These gentlemen naturally made one for their own use, and for the amusement of their friends; and the character of the instrument being thus made public, the tinmen and glaziers began to manufacture the detached parts of it, in order to evade the patent; while others manufactured and sold the instrument complete, without being aware that the exclusive property of it had been secured by a patent.

In this way the invasion of the patent right became general

among that class of individuals against whom the law is seldom enforced but in its terrors. Some workmen of a higher class were encouraged to piracy by this universal opposition to the patent; but none of the respectable London opticians would yield to the clamours of their customers, to encroach upon the rights of an inventor, to whom they were at least indebte:l for a new and a lucrative article of trade.

In order to justify these piratical proceedings, it became necessary to search out some combinations of plain mirrors, which might be supposed to have some resemblance to Dr. Brewster's instrument; and it would have been strange indeed if some theorem or experiment had not been discovered, which could have been used to impose upon the great crowd who are entirely ignorant of the principles and construction of optical instruments. There never was a popular invention, which the labours of envious individuals did not attempt to trace to some remote period; and in the present case, so many persons had hazarded their fortunes and their characters, that it became necessary to lay hold of something which could be construed into an anticipation of the kaleidoscope.

The first supposed anticipation of the kaleidoscope was found in Prop. XIII. and XIV. of Professor Wood's Optics, where that learned author gives a mathematical investigation of the number and arrangement of the images formed by two reflectors, either inclined or parallel to each other. This theorem assigns no position either to the eye or to the object, and does not even include the principle of inversion, which is absolutely necessary to the production of symmetrical forms. The theorem is true, whatever be the position of the object or of the eye. In order to put this matter to rest, Dr. Brewster wrote a letter to Professor Wood, requesting him to say if he had any idea of the effects of the kaleidoscope when he wrote those propositions. To this letter Dr. B. received the following handsome and satisfactory answer.

..

"St. John's, May 19th, 1818. Sir, The propositions I have given relating to the number of images formed by plane reflectors inclined to each other,

contain merely the mathematical calculation of their number and arrangement. The effects produced by the kaleidoscope were never in my contemplation. My attention has for some years been turned to other subjects, and I regret that I have not time to read your Optical Treatise, which I am sure would give me great pleasure. I am, Sir, your obedient humble

servant,

J. Woon."

The next supposed anticipation of the kaleidoscope was an instrument proposed by Mr. Bradley in 1717. This instrument consists of two large pieces of silvered looking-glass, five inches wide and four inches high, jointed together with hinges, and opening like a book. These plates being set upon a geometrical drawing, and the eye being placed in front of the mirrors, the lines of the drawing were seen multiplied by repeated reflections. This instrument was described long before by Kircher, and did not receive a single improvement from the hands of Bradley. It has been often made by the opticians, and was principally used for multiplying the human face, when placed between the mirrors; but no person ever thought of applying it to any purpose of utility, or of using it as an instrument of rational amusement, by the creation of beautiful forms. From the very construction of the instrument indeed, it is quite incapable of producing any of the singular effects exhibited by the kaleidoscope. It gives, indeed, a series of reflected images arranged round a centre; but so does a pair of looking-glasses placed angularly in an apartment, and so do the pieces of mirror glass with which jewellers multiply the wares exhibited at their windows. It might therefore be as gravely maintained, that any of these combinations of mirrors was a kaleidoscope, as that Bradley's pair of plates was an anticipation of that instrument. As the similarity between the two has been maintained by ignorant and interested individuals, we shall be at some pains to explain to the reader the differences between these two instruments; and we shall do this, first, upon the supposition that the two instruments are applied to geometric lines upon paper.