focus. D This lens then acts as a simple microscope, and magnifies the image as if it were seen at C D. The magnifying power of any optical instrument is the ratio of the magnitude of the image to that of the object. In a compound microscope we take the product of the magnifying powers of the object-glass and eye-glass to represent the magnifying power; thus, if object-glass magnifies 20 times and eyepiece 50, we have a power of 1,000. To obtain clear images the magnifying power ought not to exceed 500 or 600 diameters, which gives from 250,000 to 360,000 as the superficial increase. The power is greatly increased by combining several lenses. A second lens is often added to the eye-glass, for the purpose of remedying the defect arisFig. 73. ing from spherical aberration, and all the lenses are achromatic. The object, when transparent, is illuminated by a mirror, M, which concentrates the rays upon it. When the object is opaque it is illuminated by a lens which projects from the side and concentrates the rays upon it. Camera Lucida.-Invented by Woollaston in 1804. A four-sided prism is used. A is a right The rays angle, C is 135°, B and D 671° each. from an object pass in at r, are totally reflected at S, and again at o, passing to the eye, which sees an image at E, where the hand may be employed in drawing a picture of the object. This instrument is used by architects for making sketches of buildings. A Camera obscura, or dark chamber, may be constructed by using an ordinary box for a dark room. At an opening in one end is placed a convex lens, through which the rays from the object pass. They are received on a mirror placed at an angle of 45° at the opposite end of the box, and from this reflected upwards, at right angles. At the top of the box is placed a ground glass screen on which a good image E Fig. 74. of the object in front of the lens is formed. With tracing paper a drawing may be easily made on the ground glass screen. Sometimes a small circular dark room is used with a mirror at the top. There is a lens below on which the rays from the mirror fall; they are refracted and focussed on a white table in the centre of the apartment. Photographic Camera. From the camera obscura it is but a short step to the camera used by the photographer. If we imagine that a substance sensitive to light takes the place of the ground glass screen, we at once see that the object will be reproduced on the sensitive plate. This is now done on metal, glass, or paper. The photographer uses a modified form of the camera obscura. There is a brass tube in front of a box, which consists of two parts, arranged to pass one within the other. In the brass tube is a condensing lens, and at the back of the box there is a groove, in which a ground glass screen is placed and may be removed. When the object is clearly focussed on the screen, it is removed and the sensitized plate is put in the groove. The picture is instantly formed by the action of light on the chemically prepared plate; it only remains to be developed further and fixed by other chemical processes. The Spectroscope.-On a stand is placed a telescope, with object-glass covered by a cap of metal in which is a narrow slit. The light passes through this slit to the centre of the stand, on which a prism is fixed. This produces the spectrum, which is viewed by another telescope conveniently placed on the same stand. In front of the slit is a non-luminous flame, such as a Bunsen's lamp, and in this is placed a loop of platinum wire with the substance to be experimented upon. The minutest quantity of any metal will give its characteristic spectrum. The Magic Lantern.-A is a silvered reflector ; B, the light; C and D, lenses for concentrating the rays of light, and powerfully illuminating the picture; E, the picture, which must be inverted; F, a diverg ing meniscus; G, a converging meniscus, the uses of which are evident from the courses of the rays; H, the small opening in the front of the lanterns from which the light emerges. Here we see why the picture must be inverted, for at this opening the rays cross, and by inverting the picture we obtain an upright image on the screen I. Lenses C D F and G may be of different arrangement. It is not uncommon to have C and D large plano-convex lenses, and F and G smaller ones of the same kind. List of Necessary Apparatus. Ritchie's photometer (easily constructed from description in the body of this work). Rumford's photometer (easily constructed from description given). Concave and convex mirrors Ground glass plate in frame, with black and white screens Glass prisms (these may often be purchased from china dealers for a trifle) оно s. d. 5 O 5 I O IO O 5 O 020 Colour top, to show effect of combining colours (easily made) Plates of different coloured glass Complementary prisms of flint and crown glass Zoetrope to illustrate persistence Newton's disc for combination of colours List of Additional Apparatus. O Plates of quartz, aragonite, selenite, nitre, &c., each Apparatus to show laws of reflection and |