A Manual Of Photography

APPARATUS NECESSARY FOR PHOTOGRAPHY ON PAPER. 201 It is conceived that a few examples of mechanical contrivances by which the instrument is rendered portable, and in all respects convenient, will not be ont of place. Fig. 47 represents one box sliding within the other for the purpose of adjusting the focus, the lens being fitted into a brass tube, which screws into the front of the camera. The woodcut (Fig. 48) is but one box, the lens.being fitted into one brass tube sliding in another, like a telescope tube, the moveable part being ad- justed by a screw and rack. The mouth of the tube is contracted, by which any adven- titious radiations are obstructed, and a brass shade is adjusted to close the opening if required; the paper is placed in a case fitted with a glass front, as in Fig. 49, and a shutter, by which it is protected from the light until the moment it is required to throw the image upon it. In the first edition of this work, a form of camera was described, which possesses the advantages of extreme cheapness, and of being in most respects convenient. It is, therefore, here described in the language I employed in 1841:— A photographic camera should possess, according to Sir John Herschel, “ the three qualities of a flat field, a sharp focus at great inclinations of the visual ray, and a perfect achromaticity.,, There can be no doubt but these qualifications are very essen- tial,—the two first particularly are indispensable, and there is but one objection to the latter. We can only produce perfect achromaticity by a combination of glasses, and experiments prove that by increasing the thickness of the object-glass,* and the number of reflecting and refracting surfaces, we interrupt a con- siderable portion of actinism, and consequently weaken the action * The recent experiments of the Rev. Mr. Stokes most fully confirm this view.