CHAMBERS'S INFORMATION FOR THE PEOPLE. 



by the faint rays of which the image in that 

 instrument is formed ; and it was not till 1839 

 that the foundation of photography may be said 

 to have been laid, by the almost simultaneous 

 discovery, in France by Niepce and Daguerre, 

 and in England by Talbot, of the application of 

 iodide of silver, on which an image could be 

 developed, after an almost instantaneous exposure 

 to light. 



We shall first describe, briefly, the more import- 

 ant apparatus necessary for the practice of the 

 art ; then several of the modifications of the pro- 

 cesses of Daguerre and Talbot that have been 

 practised ; and, lastly, give a short account of 

 the processes now generally adopted. 



I. The Camera-obscura, or darkened chamber. 

 The invention of this instrument is ascribed to 

 Baptista Porta of Padua. Its principle is ex- 

 tremely simple, and may be illustrated thus. Let 

 a small hole be bored in a window-shutter, and 

 the room be darkened. If now the beam of light 

 entering the room by this hole be intercepted by 

 a sheet of white paper, held at a small distance 

 from the hole, an inverted image of objects with- 

 out will be seen upon the paper. By placing a 

 small convex lens over the hole, this image is 

 rendered much more distinct, or sharp, in photo- 

 graphic language. Moreover, it will be found, 

 that at a certain distance from the lens the image 

 attains a maximum degree of sharpness ; and 

 that if the paper be removed from this point to 

 any position either nearer to the lens or farther 

 from it, the image becomes indistinct and con- 

 fused. At the point of greatest distinctness the 

 image is said to be focussed. Such being the 

 principle of the camera, it is evident that in 

 practice the instrument may assume many forms, 

 provided always that it consists of a darkened 

 box or chamber having a hole at one end for the 

 insertion of a lens, or combination of lenses, and 

 at the other a screen, generally made of ground 



rig. n. 



glass, on which to receive the image. Fig. n 

 will at once give an idea of a very common and 

 simple form of camera. C is the body of the 

 instrument, made of any opaque substance ; L, 

 the tube or tubes, generally formed of brass, and 

 containing one or more lenses ; G, the obscured 

 or ground glass, upon which the image is thrown 

 for the purpose of adjusting the focus ; by 

 means of a rack behind, and the double sides 

 of the camera, the body of the instrument may 

 be lengthened or shortened till the image on 

 the ground-screen is accurately focussed. This 

 rack is frequently placed upon the tubes carrying 

 the lenses. Fig. 12 represents the camera-slide. 

 This is a thin dark box, and is used for conveying 



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a sensitive plate from the operating-room to the 

 camera, and back again after exposure. It con- 

 sists of a rectangular frame, 'made to fit exactly 

 into the back of the 

 camera when the focus- 

 sing-screen is removed. 

 At the back is a hinged 

 door, by means of which 

 the plate is introduced 

 into the slide ; and 

 in front is a shutter, 

 which is pulled up when 

 the plate is to be ex- 

 posed, and shut down ^i^Hlfl 

 after the time requisite 

 for the action of the 

 light upon the plate has Fig. 12. 



expired. It must be 



constructed so that, when substituted for the 

 focussing-screen G (fig. n), the surface of the 

 prepared plate which is intended to receive the 

 image shall correspond exactly in distance from 

 the lens with the ground-surface of the focuss- 

 ing-screen. The plate rests upon projections 

 in the interior of the slide ; and the same slide 

 may be used for plates of different size, by 

 introducing into it thin frames with holes in their 

 centre to suit the plates to be used. 



II. Lenses. One or more of these are fixed in 

 a movable tube in front of the camera. The tube 

 into which they are inserted slides light-tight 

 within another, so as to permit the lenses to be 

 brought nearer to, or removed farther from, the 

 ground-screen, so that the image may be focussed. 

 There are many requisites in a good photographic 

 lens. We can do no more than indicate some of 

 these, so as to enable any one to choose a good 

 lens for himself. 



1. When the image is focussed, it should be 

 equally sharp, or very nearly so, over all parts of 

 the ground-screen. The extent of surface over 

 which a well-defined picture will be formed, 

 depends not upon the size of the lens, but upon the 

 length of its focus. The distance between the 

 centre of the lens and the point where, for parallel 

 rays, the most perfect delineation of an object is 

 given, is the measure of its focal length. 



2. The picture obtained by a good lens should 

 be as sharply defined as the image was when seen 

 on the focussing-screen of the camera. If this be 

 not the case, it shews that the chemical and visual 

 foci of the lens are not coincident, and that it is 

 practically useless for photographic purposes. 



For the purpose of copying landscapes and 

 still-life, where time is of comparatively little 

 importance, a single achromatic lens of pretty 

 long focus suffices ; but when rapidity of action 

 becomes an object, as in portraiture, a combina- 

 tion of two achromatic lenses is necessary. By 

 this means, the light is more powerfully con- 

 densed upon the plate, and a larger aperture may 

 be allowed consistent with sharpness of definition. 

 The linear extent of a well-defined image, given 

 by any lens or combination of lenses, as well as 

 their focal depth, may be greatly increased by 

 using the diaphragm or stop. This is a thin 

 circular plate of metal or card-board, cut to the 

 size of the internal diameter of the tube containing 

 the lenses, and having a small circular hole 

 exactly in its centre. When a single lens is used, 

 the stop is placed in front of it, at an average 



