568 PHOTOGRAPHY 



Having placed the frame face downwards, carefully lay out on the glass the'object to 

 be copied, on which place the photographic paper very smoothly. Having covered 

 this with the cushion, which may be either of flannel or velvet, fix the back, and ad- 

 just it by the bar, until every part of the object and paper are in the closest possible 

 contact ; then turn up the frame and expose to sunshine. 



1618 1619 



It should be hero stated, once for all, that such pictures, howsoever obtained, are 

 called negative photographs ; and those which have their lights and shadows correct as 

 in nature dark upon a light ground are positive photographs. The mode of effecting 

 the production of a positive is, having, by fixing, given permanence to the negative 

 picture, it is placed, face down, on another piece of sensitive paper, when all the parts 

 which are white on the first, admitting light freely, cause a dark impression to be 

 made on the second, and the resulting image is correct in its lights and shadows, and 

 also as it regards right and left. 



For obtaining pictures of external nature, the Camera Obscura of Baptista Porta is 

 employed. 



1620 



The figures (figs. 1618, 1619, 1620) represent a perfect arrangement, and, at the 

 same time, one which is not essentially expensive. Its conveniences are those of fold- 

 ing (fig. 1620), and thus packing into a very small compass, for the convenience of 

 travellers. 



Fig. 1618 exhibits the instrument complete. Fig. 1619 shows the screen in which 

 the sensitive paper is placed, the shutter being up and the frame open that its con- 

 struction may be seen. 



Camera obscuras of a more elaborate character are constructed, and many of ex- 

 ceeding ingenuity, which give every facility for carrying on the manipulations out of 

 doors. 



Fig. 1621 is a section of a very useful instrument, and fig. 1622 its external form. 

 With a view to its portability it is constructed so as to serve as a packing-case for all 

 the apparatus required, a is a sliding door, which supports the lens, b, c, c are side- 

 openings fitted with cloth-sleeves to admit the operator's arms, d is a hinged door at 

 the back of the camera, which can be supported like a table by the hof>k e. f is the 

 opening for looking into the camera during an operation. This opening is closed when 

 7iecessary by the door g, which can be opened by the hand passed into the camera through 

 the sleeves c. The yellow-glass window which admits light into the camera during an 

 operation is under the door, h. i is the sliding frame for holding the focussing glass, 

 or the frame with the prepared glass, either of which is fastened to the sliding frame 

 by the check, k. The frame slides along the rod, 1 1, and can be fitted to the proper 

 focus by means of the step m. n is the gutta-percha washing-tray, o is an opening in 

 the bottom of the instrument near the door, to admit the well, p, and which is closed 

 when the well is removed by the door. The well is divided into two cells, one of 

 which contains the focussing glass, and the other the glass trough, each in a frame 



