is an eye-glass to be used by the right eye, and in the other tube, L, is an eye-glass to be used by the left eye. The tubes containing the eye-glasses are sometimes so constructed as to suit different persons whose eyes are more or less apart, and move up and down to suit eyes of different focal lengths. A great variety of very beautiful binocular pictures have been taken photographically, suitable for the stereoscope. These pictures are called "slides." If we put one of these slides into the horizontal opening s, and place ourselves behind R L, we shall see, by looking through R with the right eye, and L with the left eye, the two pictures on the slide combined in one, and in the same relief as the object or scene they represent.

The name "stereoscope" is derived from two Greek words, one of which signifies "solid," and the other "to see."

B

The lenses of the stereoscope are formed by cutting a double-convex lens ABCD (Fig. 95) in two by a plane, B D, passing through the centre of the lens. Two optically eccentric lenses are then cut out of these, so c that the diameter of each shall be equal to the radius of the original lens. A section of the original lens is shown in Fig. 96, from which it will appear that the two optically eccentric lenses A E, E C will have their thickest part at E, and their thinnest at A and c, while the geo

D

Fig. 95.

E

Fig. 96.

metrical centres are at F and G.

If the two lenses A E, EC be set with their edges A and c towards each other in the two eye-holes R, L (Fig. 94), whose distance apart is equal to that of the

eyes, and let a slide be placed before them at a distance equal to their common focal length, the two pictures on the slide will be made to coincide and unite in one by the refraction of the lenses, and the eyes will see the combined picture in stronger relief than if the original object were placed before them.

The author of this treatise has recently seen a new design of a binocular stereoscope, made at the manufactory of Mr. Meagher, 21, Southampton Row, London, which is likely to become very popular, as it can be sold at a very low price, and thus brought within reach of the great mass of the people, affording them at the same time both innocent recreation and instruction of the highest order.

Revolving or Magazine Stereoscope.-This is an arrangement for the exhibition of a number of slides, varying from 25 to 100, according to the price, at evening parties and social gatherings. By means of a handle, which revolves, a fresh picture is presented to the observer at every half-turn of the handle.

The Graphoscope. This is an optical instrument, shown in perspective in Fig. 97, for viewing large and small photographs, stereoscopic slides, drawings, medals, engravings, and other objects of art. The most minute details are caused to appear singularly clear and vivid. It further serves as an easel, upon which any work requiring a magnifying glass may be executed.

То open it for viewing objects, raise the platform -under which is the large lens sliding on two brass rods to the inclination desired. Turn the lens to the position in the diagram, and adjust it to a suitable height. Raise the easel perpendicular with the platform, and place the object upon it. The brass arms can extend the field of view. The required focus

is obtained by moving with the two hands the easel along the groove through the middle of the board towards the lens.

To convert it into a stereoscope, put back the lens

Fig. 97.

into its place under the platform, and raise up the frame with the two stereoscopic lenses. Move the easel so as to suit the vision. This is all that is required for opaque slides. For transparent slides, raise the folding camera which is under the frame. A small ring with elastic band fastens the camera to the frame, and makes it stand firm. Place the transparent object on the easel, and obtain the focus as before. Transparent objects require good light immediately behind the easel; by day the sunlight; by night a lamp.

The Kaleidoscope.-This optical instrument, invented by Sir David Brewster, is for the purpose of producing, in endless number and variety, beautiful forms, and exhibiting them so that they may be copied and rendered permanent in various kinds of manufacture. It consists of two oblong pieces of looking-glass placed edge to edge, and inclined to each other at an angle of

60°. Thus arranged, they are fixed in a tube of tin or brass of suitable size, an end view of which is shown in Fig. 98, where the circle A C B represents the tube, and A B and A C the ends of the pieces of glass. One end of the tube is covered with two discs of glass, between which objects are placed loosely, so that they can fall from side to side and assume an infinite variety

B

Fig. 98.

of casual positions. The outer disc is of ground-glass, to prevent the effect being marred by the view of external objects. The other end of the tube is covered by a diaphragm with a small eye-hole in its centre, through which the observer looks at the objects contained in the cell between the discs at the other end. Not only are the objects seen, but their reflection also in each of the inclined glasses; and when the angle of inclination is 60°, the object will be seen five times repeated, in positions symmetrically disposed round the

line formed by the edges at which the glasses touch each other.

The observer, looking through the eye-hole of the kaleidoscope, sees a circle whose apparent diameter cc" is twice A c, the breadth of the looking-glass. The circle is divided into six angular spaces, two of which are the first reflections, and other two the second reflections of the inclined glasses. The other two consist of the actual space included between the glasses, and a similar space opposite to it, which receives at the same time the third reflection of both glasses, which overlie each other and appear as only one image.

For the purpose of extending the power of the kaleidoscope, and introducing into symmetrical pictures external objects, whether animate or inanimate, the inventor applied a convex lens L L (Fig. 99), by means

of which an inverted image of a distant object, N N, is formed quite close to A B C, the ends of the reflectors. In this construction the lens is placed in one tube and the reflectors in another; so that by pulling out or pushing in the tube next the eye E, the images of objects at any distance can be formed at the place of symmetry, that is, near the ends of the reflectors.

Varieties of Form. The kaleidoscope may be constructed with the reflectors inclined at any angle which is in an aliquot part of 360°.

The Quadrant is an optical instrument, used by officers in the royal and merchant navies, for the pur