CARE OF LENSES.
The lens is the most valuable and deli cate part of the whole photographic outfit, and a small case should be provided for it, to prevent it being knocked about and to protect it from the dust. The less dusting a lens has consistent with cleanliness the better ; any dust that may get on_it should be carefully wiped off with an old soft silk handkerchief, previously washed in clear water and dried away from dust, as the slightest speck of dust on the handkerchief may scratch the beautiful surface of the glass. The silk handkerchief used for When the lens is not in use, the ban( should be kept over the slot in order to exclude both dust and air. Ar Iris diaphragm should be chosen it preference to either Waterhouse or ro tating stops, as the Iris is undoubted!, superior in every way, and well worth the extra money charged for it. With them there are no loose parts liable to be lost and the aperture can, if desired, b altered during the exposure withou fear of vibration. After use the lens from the camera, and put i away in its case. The temperature o; the lens should not be below that of dusting the lens must not be used for any thing else. The lens case may be made of cardboard and lined with chamois leather. It can be made either at home or by a professional casemaker. The
latter should have proper measurements given him, as the lens should not go out of the owner's hands, if be values it as he ought. Sometimes the combinations of a -rectilinear lens will require dusting be tween the glasses. To do this, only one combination at a time should be un screwed, and, when dusted, should be re placed before the other is taken out, so as to guard against mixing them. It was at one time customary, when the lens was fitted with Waterhouse stops, to keep a flat indiarubber band around the lens tube, so that when using the lens at full aper ture, the band could be slipped over the diaphragm slot, and so prevent light enter ing. Nowadays, however, this is seldom necessary, as lenses are almost invariably supplied with a " full aperture stop." This, although not really a stop, effectu ally stops any extraneous light entering.
atmosphere in which it is to be used, otherwise the vapour in the air will con dense on the glass and produce fog in the negative. Should this occur, and some times in a cold studio in the winter it is unavoidable, the lens should be slightly warmed by the fire, unscrewing one lens so that the air may escape.