The spectres designed by Robin also attracted considerable public attention within recent times. They were images formed by the medium of transparent glass. Glass panes often produce the phenomenon of spectres. In the evening, when it is dark out of doors, it is easy to prove that the reflection of objects in a lighted room can be reproduced behind the window panes by reason of the darkness outside. If we approach the window pane, we see also the real objects outside, a balcony, tree, etc. These real objects mingle with the reflected image, and combine to produce very curious effects. In this way M. Robin has contrived the effects of the theatre. He throws on the stage the reflection of a person dressed as a Zouave, and he himself, armed with a sabre, stabs the spectre through the body. A great number of other singular effects have been obtained in the same manner. Pepper’s Ghost was managed in this way. [Illustration: Fig. 145.—The ghost effect.] Within recent times, images produced in a similar way have been utilized to facilitate the study of drawing. A piece of glass is fixed vertically on a black board (fig. 146). A model to copy from is placed on one side of the piece of glass, and is arranged so that the visual ray passes obliquely through the glass, and we perceive the reflection of the design very clearly on the other side. It is then very easily reproduced with a pencil on a sheet of white paper by tracing the outlines. [Illustration: Fig. 146.—Drawing by reflection.] CHAPTER XIII. VISION—THE EYE—THE STEREOSCOPE—SPECTRUM ANALYSIS—THE SPECTROSCOPE—THE TELESCOPE AND MICROSCOPE—PHOTOGRAPHY—DISSOLVING VIEWS—LUMINOUS PAINT. The eye and vision are such important subjects to all of us that we may be excused for saying something more concerning phenomena connected with them, and the instruments we use for assisting them. We do not propose to write a treatise upon the physiology of vision, for we know the image in the eye is produced physically in the same manner as the image in a camera obscura. In the eye the sides of the box are represented by the sclerotic (see chap. x. fig. 95); the dark inner surface has its parallel in the pigment of the choroid; the opening in the box in the pupil of the eye; the convex lens in the crystalline and the cornea; and the retina receives the image. But why we see—beyond the fact that we do see—no one can explain. Science is dumb on the subject. Thought and consciousness elude our grasp; and, as Professor Tyndall says on this subject, “we stand face to face with the incomprehensible.”