The earliest writer who appears to have described the use of two plane mirrors, was Baptista Porta, who has given an account of several experiments which he performed with them, in the second chapter of the seventh book of his Magia Naturalis.
As the combination of plane mirrors which he there describes has been represented as the same as the Kaleidoscope, we shall give the passage at full length:—
Speculum è planis multividum construere.
“Speculum construitur, quod polyphaton id est multorum visibilium dicitur, illud enim aperiendo et claudendo solius digiti viginti et plura demonstrat simulacra. Sic igitur id parabis. Ærea duo specula vel crystallina rectangula super basim eandem erigantur, sintque in hemiolia proportione, vel alia, et secundum longitudinis latus unum simul colligentur, ut libri instar apte claudi et aperiri possint, et anguli diversentur, qualia Venetiis factitari solent: faciem enim unam objiciens, in utroque plura cernes ora, et hoc quanto arctius clauseris, minorique fuerint angulo: aperiendo autem minuentur; et obtusiori cernes angulo, pauciora numero conspicientur. Sic digitum ostendens, non nisi digitos cernes, dextra insuper dextra, et sinistra sinistra convisuntur; quod speculis contrarium est: mutuaque id evenit reflexione, et pulsatione, unde imaginum vicissitudo.”
—Edit. Amstelod, 1664.
The following is an exact and literal translation of this passage:—
How to construct a multiplying speculum out of plane ones.
A speculum is constructed, called polyphaton, that is, which shows many objects, for by opening and shutting it, it exhibits twenty and more images of the finger alone. You will therefore prepare it in the following manner:—Let two rectangular specula of brass or crystal be erected upon the same base, and let their length be one and a half times their width, or in any other proportion; and let two of their sides be placed together, so that they may be opened and shut like a book, and the angles varied, as they are generally made at Venice. For by presenting your face, you will see in both more faces the more they are shut, and the less that the angle is; but they will be diminished by opening it, and you will see fewer as you observe with a more obtuse angle. If you exhibit your finger, you will see only fingers, the right fingers being seen on the right side, and the left on the left side, which is contrary to what happens in looking-glasses, and this arises from the mutual reflexion and repulsion which produce a change of the images.[20]
It is quite obvious, from the preceding passage, that the multiplying speculum described by Porta was not an invention of his own, but had been long made at Venice. In the very next chapter, indeed, where he describes a speculum theatrale or amphitheatrale (the show-box of Harris and other modern authors), he expressly states, that a multiplying speculum was invented by the ancients.—“Speculum autem è planis compactum, cui si unum spectabile demonstrabitur, plura illius rei simulachra demonstrabit, prudens invenit vetustas; ut ex quibusdam Ptolemæi scriptis quæ circumferuntur percipitur;” that is, “a speculum consisting of plane ones, which, when one object is presented to it, will exhibit several images of it, was invented by skilful antiquity, as appears from some of the writings of Ptolemy.” This speculum, which consists of several mirrors arranged in a polygon, with the object within it, is characterized by Porta as puerile, and much less wonderful and agreeable than one of his own, which he proceeds to describe. This new speculum consists of ten mirrors, placed within a box, in a sort of polygonal form, with one of the sides of the polygon open. Architectural columns, pictures, gems, pearls, coloured birds, etc., are all placed within the box, and their images are seen heaped together in inextricable disorder, as a whole, but so as to astonish the spectator by their number, and by the arrangement of individual groups. Porta speaks of this effect as so beautiful, “ut nil jucundius nil certe admirabilius oculis occurset nostris,”—that nothing more agreeable, and certainly nothing more admirable, was ever presented to our eyes.
It would be an insult to the capacities of the most ordinary readers to show that the instruments here described by Baptista Porta have no farther connexion with the Kaleidoscope than that they are composed of plane mirrors.