A Manual Of Photography

202 PRACTICE OF PHOTOGRAPHY. on the photographic material, whatever it may he ; but our opticians have succeeded to a great extent in overcoming this difficulty. We may, to a considerable degree, get rid of the defects arising from chromatic dispersion, without having recourse to a combination of glasses of different refracting powers. I have long used myself, and constructed for others, a camera obscura, which appears to answer remarkably well, with a non-achromatic lens. Fig. 50 a represents the aperture of the lens ; z'z', a box sliding into an outer case, h h' ; k k, a third division, containing a ground glass at the back, and a door which can he raised or lowered by the screw F, the whole fitting into the frame h h'. Figure 51 is a section of the camera, a is a lens of a peri- scopic form, whose radii of curvature are in the proportion of 2 to 1. This meniscus is placed with its convex surface towards the plane of representation, and with its concavity towards the object. The aperture of the lens itself is made large, hut the pencil of rays admitted is limited by a diaphragm, or stop, constructed as in the figure at 6, between it and the plane of representation, at about one-tenth of the focal length /. from the lens. By this arrangement ob- jects are represented with considerable distinctness over every part of the field, but little difference being observable be- tween the edges and the centre, e is the plate of ground glass at the hack, which serves to adjust the focus by, and also to lay the photographic paper on, when we de- sire to copy any object ; d, a door to shut s off the light from the paper or plate un- til the moment we desire to expose it to 52. luminous agency. Fig. 52 represents this