“From both these contrivances, and from every optical instrument with which I am acquainted, the Kaleidoscope appears to differ essentially, both in its effect and in the principles of its construction. “As to the effect, the thing produced by the Kaleidoscope is a series of figures presented with the most perfect symmetry, so as always to compose a whole, in which nothing is wanting and nothing redundant. It matters not what the object be to which the instrument is directed; if it only be in its proper place, the effect just described is sure to take place, and with an endless variety. In this respect the Kaleidoscope appears to be quite singular among optical instruments. Neither the instrument of Bradley, nor the experiment or theorem in Wood’s book, have any resemblance to this; they go no further than the multiplication of the figure. “Next, as to the principle of construction, Dr. Brewster’s instrument requires _a particular position of the eye of the observer, and of the object looked at_, in order to produce its effect. If either of these is wanting, the symmetry vanishes, and the figures are irregular and disunited. In the other two cases, no particular position, either for the eye or the object, is required. “For these reasons, Dr. Brewster’s invention seems to me quite unlike the other two. Indeed, as far as I know, it is quite singular among optical instruments; and it will be matter of sincere regret, if any imaginary or vague analogy, between it and other optical instruments, should be the means of depriving the Doctor of any part of the reward to which his skill, ingenuity, and perseverance, entitle him so well. “JOHN PLAYFAIR, _Professor of Natural Philosophy in the University of Edinburgh_.” “_P.S._—Granting that there were a resemblance between the Kaleidoscope and Bradley’s instrument, in any of the particulars mentioned above, the introduction of coloured and movable objects, at the end of the reflectors, is quite peculiar to Dr. Brewster’s instrument. Besides this, a circumstance highly deserving of attention, is the use of two lenses and a draw-tube; so that the action of the Kaleidoscope is extended to objects of all sizes, and at all distances from the observer, and united, by that means, to the advantages of the telescope. “J. P.” Professor Pictet’s opinion is stated in the following letter:—