Fig. 66.—Diorama.
Fig. 66 will show the way in which these changes are managed. The large picture, which is hanging vertically, is painted both in front and behind. The front is illuminated by reflection from a semi-transparent screen placed over it, which receives the light of the floor above. The back is lighted from the windows behind, which are provided with blinds to regulate the amount of light. The effects produced by the diorama were truly marvellous, and Daguerre had a special talent for this kind of painting. His famous Midnight Mass, which was exhibited at the Regent’s Park, was one of the most renowned of his works. The scene first represented a dark, empty church, feebly lighted by a small altar lamp, but gradually the lights appeared here and there, worshippers congregated in front of the altar, filling the nave and aisles. In Paris the same scene was exhibited, representing the interior of the Church of St. Germain l’Auxerrois with such perfect reality, that a countryman actually threw a halfpenny against the painted canvas, to see whether he were really in a church or not.
The next scene represented the destruction of the village of Goldau, near Lucerne, by a landslip. First there appeared a smiling fertile valley, its sides crowned with verdure; a storm gradually rose, the rain fell, the wind blew, the lightnings flashed, and the thunder rolled in the distance. Darkness at last closed in, and when the sun once more rose over the valley, nothing was to be seen but a mass of fallen rocks.
CHAPTER VII.
THE STEREOSCOPE.
Having devoted so much space in the preceding chapters to optical amusements of a purely recreative character, it is only right that we should now say a few words on certain instruments of a less frivolous character than those we have lately been considering, and which deserve at our hands the most serious attention. We shall, therefore, in the present chapter, speak of an ingenious instrument which serves to show in relief the images of objects depicted on a flat surface. We have already seen, that although we have two eyes, provided with lenses and screens by means of which the images of things around us are formed, we only perceive a single object; and the student has no doubt long since wondered why nature has bestowed two eyes upon us, when only one would have apparently served the same purpose. This question was for a long time a complete puzzle to philosophers, and it was not until Professor Wheatstone made his experiments on binocular vision in 1838, that the matter received a satisfactory explanation. He showed that each eye receives a different impression of any object upon the retina, and that it is in consequence of the union of these slightly dissimilar images that the sensation of relief is experienced. A one-eyed man or a Cyclops would only partially perceive relief in the objects presented to his view, in consequence of a single image being sent to his brain. He would, no doubt, after examining the things he saw with his hands, know they were solid, and generally see them so; but if a new object were presented to his view he would have some difficulty in knowing whether it had a flat surface or not.
Fig. 67.
The principle of binocular vision may be explained as follows: If a playing die, such as is represented in fig. 67, be held out at arm’s length in the position indicated in the figure, and looked at first with the left eye and then with the right, we shall find that in the first case we see a little of the three dots on the left-hand side, and in the second we lose sight of the three dots and see a little of the single one on the right-hand side. The images seen by each eye are, therefore, slightly dissimilar, and it stands to reason that, if by any means we can combine two slightly dissimilar flat pictures of a solid object, we shall see it in relief. This was proved practically by Professor Wheatstone, who constructed an instrument capable of effecting the desired union, and which has since been called the stereoscope, from two Greek words signifying ‘to see solid.’ The instrument remained for a long time fallow, so to speak, from the difficulty of drawing two pictures that should be identical in size and details, although dissimilar in the arrangement of their perspective. It was, therefore, not until photography enabled us to do this with the greatest ease and exactitude that the stereoscope became common. The instrument first devised by Professor Wheatstone, was what is termed a reflecting stereoscope, and was expensive to make and cumbrous to use. It was modified by Sir David Brewster, by the substitution of prisms for reflectors, and was thus made cheaper and more portable. The refracting form of stereoscope is so familiar to most people, that it really needs no description. It will only be necessary to mention that the prisms used in the eye-pieces are made by cutting a double convex lens in two, and reversing the halves. They are so placed that the centre of each prism is just in the centre of each eye; but as the eyes of different people vary in distance, an arrangement is generally added so that the eye-pieces may slide from side to side. Being cut from lenses, the prisms have a magnifying power; consequently other means are provided for sliding them up and down to suit the length of focus in different eyes.
