276 HOW TO WORK 



bore fixed in a wooden base, has at opposite sides at the exact 

 centre of the microscope or camera tube, two apertures cut | of 

 an inch in diameter, and two tubes i inch in length soldered in ; 

 in these short tubes slide the tubes of two funnels of tin, 3^ 

 inches deep and 2^ in width, blackened inside. Again at right 

 angles to the apertures of the two short tubes are two circular holes. 

 Against the outer rim of the funnel nearest the stage is placed the 

 plano-convex condensing lens, and against the rim of the opposite 

 funnel a hemispherical concave reflector with a central aperture. 

 Beneath the short tube carrying the first funnel a portion of the 

 eight-inch tube is cut tongue shape and turned in to support a 

 narrow spirit-lamp. An arrangement is made in the support holding 

 the reflector by which a small tube can be passed through the central 

 hole in the reflector, and by allowing a weight to fall steadily a short 

 distance, a wire piston is carried along the tube, and projects the 

 short piece (3 inches) of magnesium wire as it burns away, 

 the ignited point being in the centre between the two funnels 

 and at the foci of the condensing lens and silvered reflector. 

 This plan requires a little experience to allow the necessary motion 

 of the hand to compensate for the rate of burning, and might be 

 constructed as self-acting, but practically it answers very well, and 

 is easily made. 



354. Photographs of Microscopic Objects for the Magic Lantern. 

 Although no means are yet known by which a minute object, 

 magnified by the higher powers of the microscope, can be thrown 

 upon a screen so as to be seen by a number of persons at once, 

 almost the same result has been obtained by magnifying a photo- 

 graph of the object in an oxy-hydrogen magic lantern. It may not 

 be misplaced to say a few words on the negatives best suited for 

 enlargement, and the mode of enlarging to a moderate size. The 

 negative should be clear without stains, and if containing only a single 

 object or objects separated, the field should be only sufficiently 

 dense not to allow any light to pass through in the period of time 

 necessary to secure a reversed copy or positive on glass. To effect 

 this there are several ways. If to be of the same size, a sensitised, 

 albumenised, or tannin prepared plate, dry, has the negative laid 

 carefully face down on the prepared surface, and fixed as in a 

 printing frame, or held very tightly, then exposed to ordinary day- 

 light for a few seconds, or else for a longer period opposite a fish-tail 

 gas-light. When impressed the negative is removed, and the image 

 developed in the manner employed for the kind of film used. A 

 very fine deposit is requisite, therefore the development should be 

 gradual ; great diversity of tone is procured by using various 



