Fig. 306.
Nos. 1 and 2 are the discs. No 3. Kalotrope in elevation. No. 4. Side view of kalotrope, showing the multiplying wheels and the perforated and painted discs moving in opposite directions.
XI. The Photodrome.
This is a second optical arrangement by Mr. Rose for showing spectral illusions; and it is superior to the last, inasmuch as it offers to the public lecturer a most effective means of presenting these deceptions to a large audience. It differs from the kalotrope in several important points. It dispenses with the discs of apertures, and leaves the device disc with its face fully exposed to the spectators. The effects are produced by a powerful light, thrown through the tube of a lantern, and broken by a wheel working across it. The apparatus, as it at present stands in the inventor's possession, consists of two distinct parts; the one a movement for the device discs, and the other for the light. A wheel four feet in diameter is connected with a train of movement capable of giving it five hundred or six hundred revolutions per minute. On this wheel the device disc is placed, in full view of the spectators, and set in motion. From an opposite gallery the light is thrown, and broken by a wheel of such diameter and number of apertures as will admit the velocity of the photodrome (or light-runner) to be at least six times the velocity of the device disc; whilst the apertures are of such width as to restrict the duration of the light-flash to about one-two-thousandth of a second. The wheel working across the light has a train of movement for raising the velocity to two thousand revolutions per second. The management of the apparatus is very simple. The device-wheel is brought to a steady, rapid rotation, and the operator on the light then works his wheel with gradually increasing velocity, until he overtakes the figures of the device, where, by mere delicacy of touch, he is able to hold them stationary or give them motion, at pleasure.
Theories of light and colour still agitate the scientific world, although that man must be bold who will assert that his hypothesis is fitted to explain every difficult point that arises as our experimental knowledge increases. Mr. G. J. Smith, of the Perth Academy, has propounded a very ingenious theory of light and colour, supported by some clever experiments. But, as Solomon says, "there is nothing new under the sun," and in an able paper Mr. Rose, of Glasgow, lays claim to the anticipations of Mr. Smith's theory as follows:—
"My attention has been directed to a paper entitled 'The Theory of Light,' by G. John Smith, Esq., M.A., of Perth Academy. I think it is now nearly two years since I communicated an interesting fact to Professor Faraday, and to a member of our local Philosophical Institution, which may fairly claim to have anticipated Mr. Smith's theory. The fact was this: that if a piece of intensely white card be held in one hand, with the light of a powerful gas-jet falling upon it, and if the other hand has command of the gas-tap, as the light is gradually reduced, the card will assume the prismatic colours down to intense blue, and as the light is restored the colours will present themselves in inverse order. The experiment showed, very conclusively to my mind, that light is homogeneous, and that what we name colour is only the various affection of the optic nerve by a greater or lesser radiation of light from a focal point in an imperfect reflector—say, in the instance, a white card. I apprehend that Mr. Smith confuses his theory when he speaks of alternations of light and shadow producing colour. Shadow, or darkness, is mere negation of light. We do not see mixtures of light and darkness, or blackness and whiteness, but light in its several degrees of intensity. Mr. Smith's experiments present only what my kalotrope has done, and what my later device, the photodrome (now nearly three years old) is doing in a much more perfect manner. It is one of the mysteries intelligible only to the initiated, that whilst Mr. Smith's paper seems to have been received with great favour by the British Association, my communication relative to the photodrome was voted 'not sufficiently practical.'
"Since I have come before the public with an experiment, which in any view is an interesting one, permit me to reproduce it under several distinct conditions, and to add a brief narrative of remarkable presentations of colour that have come before me, and which, so far as I am aware, are perfectly novel, or known only through the more recent experiments of Mr. Smith. Professor Faraday very courteously acknowledged my communication of the experiment with the card, but said that it only partially succeeded with him, and added that probably this was owing to some decay of sensitiveness in his eyes. More likely I failed to state with sufficient clearness the conditions of the experiment, since I have always found nine persons out of ten perfectly agreed as to the effects produced when they have been at my side. The transitions from white to yellow, orange, red, and thence to intense blue, are, I may say, invariably admitted. Success depends on a very slow and regular reduction and restoration of the light. I have given one method of performing the experiment, and will add other two. Allow the light to remain undisturbed, and begin by holding the card near to it; then keep the hand steady and the eye intently fixed upon the card, and retire gradually with your back to the light, and the colours will change in the order of the prismatic spectrum from yellow to intense blue. On returning backwards towards the light the colours will again present themselves, but in inverse order. In this form of the experiment we are certain that the light remains precisely the same throughout. The third method is this: Place a circle of white card, about three inches in diameter, in the centre of a black board, and let a spectator stand within twelve inches of the board, with his eyes fixed upon the card. Let an operator be provided with a light so covered that it shall not fall on the eye of the spectator; then, as he retires with the light or returns with it, the spectator will see the colours as before. This arrangement evidently subjects the experiment to a severe test, since the black board enhances the whiteness of the card, and tends to preserve it.... Whilst pursuing my principal object, I frequently noticed most remarkable presentations of colour; but, as the conditions were for the most part unsuitable to the lecture-room, I gave them only a passing regard. Allow me to instance a few of the experiments.
"The first refers to the kalotrope, which may be briefly described as an arrangement of two concentric wheels, working nearly in contact and in contrary directions. Discs of various devices are provided for the hinder wheel, and a number of perforated black discs for the one in front. When a disc charged with twelve black radii is placed on the hinder wheel, the six spokes of the front wheel, in passing rapidly across it, convert the twelve black radii into twenty-four apparently stationary white radii upon a tinted ground. Here is a remarkable presentation of the complementary, inasmuch as it is placed permanently before the eye by persistence.