533 



CAMERA OBSCURA. 



CAMP, ROMAN. 



63* 



2. When the object Q is near the prism, the latter is raised or' 

 lowered on its pillar until the paper and the object are at equal or nearly 

 equal distances from the prism. The image of Q will then be thrown 

 on the paper without any necessity for a lens. 



The camera lucida thus described presents the objects direct, and 

 without reversal of left and right ; but it is by no means an easy 

 instrument to use. There is what is called a knack, which some can 

 acquire, and some cannot. It has been found of considerable use by 

 travellers who were unable to draw from nature, but it is at best little 

 better than a toy, and thus it has been in a great measure superseded 

 even for unartistic travellers by the photographic camera. 



CAMERA OBSCURA, the dark chamber, an instrument by means 

 of which images of external objects are thrown upon a plane surface, 

 or, as it is termed, focussing screen. 



The earliest description of the camera obscura is in the seventeenth 

 book of the ' Magia Naturalis ' of John Baptista Porta, who directs to 

 make a small aperture in the shutter of a perfectly dark room, and to 

 let the light fall upon a white screen or wall. It is evident that in 



thi.-i way inverted images will be obtained, the magnitude of which 

 will depend upon the distance of the screen from the aperture. Porta 

 makes one more step towards the present instrument. He states, as 

 a secret which he had concealed till then, and had intended always to 

 conceal, that if a convex glass be applied to the aperture, all external 

 objects shall be seen as clearly as to a bystander, with so much 

 pleasure that those who see it can never enough admire it. He does 

 not appear to have found out that the screen should be curved, in 

 order that the points of convergence of all the pencils should fall 

 exactly upon it. The present form of the common camera obscura 

 is as follows : The box in the diagram has a tubular lateral opening 

 which should, if the lens be large, be partly closed by a diaphragm to 



Fig. 2. 



diminish the effective aperture. The rays passing through the lens 

 from a distant object are prevented from converging by meeting a 

 mirror inclined at 45 to the bottom of the box, and are thus made to 

 converge at or very near to the glass plate A, one side of which is 

 ground. A shade over the glass plate excludes direct light from 

 falling on it. The under side of the glass plate should be curved ; 

 but, generally speaking, the aperture is so small that this is of little 

 consequence for ordinary purposes. The tube in which the lens is 

 placed in made to slide, so that it can be adjusted to near or distant 

 object*. The instrument l>eing adjusted, the landscape directly in 

 front of the object-glass will be painted on the glass A and not inverted, 

 luft. only the right side on the left, and vice vertd. 



A camera obecura for exhibition, such as is now most commonly 

 seen at summer watering-places, is generally made in a room with a 

 conical roof and an aperture at the top. Above this aperture is a 

 revolving plane mirror inclined at 45, and reflecting pencils down- 

 wards. A convex lens caused these pencils to converge upon a surface 

 <>!' pl.-istt-r of Paris, properly curved. The mirror revolves about a 

 vertical axis, thus allowing all the compass-points of a landscape to be 

 successively thrown on the surface. 



On these instruments we may observe that they are nrtijktal eyes. 

 The ray are collected by a lens or prism, and are then made to paint 

 images upon a surface, which answers to the retina. Or conversely 

 we may say that, in the common experiment of cutting away part of 

 the eye of a dead animal to show the images on its retina, the eye 

 thus employed is converted into a camera obscura. 



Prior to the discovery of photography, the camera obscura was 

 only an elegant scientific toy ; but the important part it was made to 



play in that beautiful art at once raised it to the rank of a philosophic 

 instrument, and its adaptation to the peculiar requirements of the art 

 has been a subject of continuous and careful investigation. In form, 

 as in principle, the photographic camera differs little from that above 

 described. But the only light admitted into the camera must pass 

 through the lens, and the image, instead of being reflected by a mirror 

 on to the horizontal ground-glass screen A, is thrown at once upon the 

 screen itself, which is upright and corresponds to the back of the camera 

 in Jig. 2, or to the screen B in fij. 1. The screen (in photographic 

 cameras called the focussing screen) is of ground glass, as in the 

 ordinary camera ; but in the practice of photography it is only used 

 for ascertaining that the object is correctly placed and properly denned, 

 when it is removed, and the prepared plate or paper substituted. 



The great objects to be obtained in photographic cameras are, that 

 the images shall be exhibited on the screen clearly and sharply defined : 

 iu their true forms and with their natural light and shade, on a flat 

 plane, that is, and free from all aberration or distortion. In order to 

 effect this, the nicest adjustment of lenses, prepared with the strictest 

 accuracy, are employed. In practice, it is found that only achromatic 

 combinations, in which the chemical and optical foci are coincident, 

 will serve j and that for near objects, as portraits, and for objects 

 which are situated in varying planes, as in landscapes, very different 

 combinations must be used. For portraits, the usual combination is 

 formed by two compound achromatic lenses, the two flint lenses being 

 turned towards each other, and the convex side of the front com- 

 bination towards the sitter. For landscapes, a meniscus lens, formed 

 of a double convex lens of plate-glass cemented to a double concave 

 lens of flint-glass, is employed, the concave side (which should be very 

 slightly concave) being turned towards the interior of the camera. 

 The varieties of lenses made by different opticians are, however, very 

 great, but a description of them would be out of place here. Among 

 the most successful are the orthoscopic, the Petzval, the aplanatic, &c. 



The varieties of photographic cameras are even more numerous than 

 of photographic lenses ; but none of the varieties involve any material 

 difference of principle. The most usual varieties are such as the 

 folding, the expanding, and the adjusting camera, or some combination 

 intended by the inventor to secure greater accuracy or portability, or to 

 avoid some defect in cameras in ordinary use. For taking stereographs, 

 however, and for copying photographs, prints, &c., on a larger or 

 smaller scale, cameras of a special character are manufactured. The 

 Stereoscopic Camera is, however, little more than a double camera, or 

 two cameras combined in one (aud hence sometimes called a double 

 lens, or binocular camera) with adjustments for giving slight inde- 

 pendent motion to each for rapidly covering the lenses, &c. The 

 Cnjii/iiii/ Camera is a more complex apparatus ; arrangements having to 

 be provided for lengthening the body of the instrument before as well 

 as behind the lens, altering the place of the lens, focussing-glass, 

 camera-back, dark frame, diaphragms, &c., according to the size of the 

 copy, whether the original is transparent or opaque, &c. Cameras 

 intended for obtaining " instantaneous photographs," as well as for other 

 special purposes, have also been manufactured, among which perhaps 

 might be mentioned Mr. Skaife's ingenious ' pistol camera." But for 

 further particulars we must refer the reader to treatises on photography 

 and to the articles LENS and PHOTOGRAPHY. 



CAMERONIANS. [CAMERON, RICHARD, in BIOG. Div.] 



CAMOUFLET, or STIFLER, is a term applied in military mining 

 to a small charge of powder, or undercharged mine, which makes little 

 or no crater above ground, but by the concussion underground destroys 

 a portion of the enemy's gallery and suffocates his miners. 



CAMP. [ENCAMPMENT.] 



CAMP, ROMAN. There are few parts of the art of war as prac- 

 tised by the Romans in which the laborious and regulated carefulness 

 of that sagacious people is more evident than in their system of castra- 

 metation. In the present article we shall give some account of the 

 construction of their camps, and the arrangement of the troops in 

 them. 



In speaking of the Roman camps, it is necessary to distinguish 

 between the summer and winter encampments (castm cestiva et /ty- 

 bema) ; and again to discriminate between those camps which were 

 formed to protect the army for a short period, aud those which they 

 proposed to occupy for a longer time, which were called castra s/atira. 

 The difference between these consisted chiefly in the strength of the 

 fortifications, and in the superior size of the temporary camps, which 

 were intended commonly for the whole army, while the more perma- 

 nent encampments were for divisions of the army. Winter encamp- 

 ments were not used by the Romans in the earlier periods of their 

 history, when their chief wars were little more than summer cam- 

 paigns, and were waged against neighbouring nations : but in a later 

 age, when permanent conquest was their aim, and the war continued 

 several years, the army was regularly distributed into winter-quarters, 

 and often spread over a considerable extent of country, iu order to 

 overawe the subjugated districts, or because forage and provisions 

 could be obtained by the army in several divisions more easily than 

 when it was in one body. Caesar in hia Gallic campaigns regularly 

 distributed his army into winter-stations, so strongly fortified that 

 though several attempts were made upon them only one was taken, 

 and that because the commander unwisely abandoned it. (Cies. <r > 

 Bel. Gal.,' lib. v.) 



De 



