Careful laboratory tests performed for the U. S. Air Service showed that neither low temperature nor low pressure, such as would be met at high altitudes, alter the focus of any ordinary lens by a significant amount, and that the possible contraction of the camera body was of negligible effect on the focus (not more than 1/200 per cent. per degree centigrade with a metal camera). In complete harmony with these tests has been the experience that if the ground focussing is done carefully, by accurate means, then the air focus is correct. The whole matter thus becomes one of precision focussing. The best method, applicable if the air is steady, is to focus by _parallax_. The ground glass focussing screen is marked in the center with a pencilled cross. Over this is mounted, with Canada balsam, a thin microscope cover-glass. The camera is directed on an object a mile or more away, and the image formed by the lens is examined by a magnifying glass through the virtual hole formed by the affixed cover-glass. With the pencil line in focus the head is moved from side to side. If the image and pencil mark coincide they will move together as the head is moved. If the image moves away from the pencil mark and in the _same_ direction as the eye moves, the image is too near the lens. If the image moves away in the _opposite_ direction to the motion of the eye, it is too far from the lens. In either case the focus is to be corrected accordingly. In place of a distant object, which may waver with the motion of the air, we may use an image placed at infinity by optical means. The _collimator_, an instrument for doing this, consists of a test object (lines, circles, etc.) placed accurately at the focus of a telescope objective. The camera lens is placed against this and focussed by parallax, as with a distant object. Collimators are employed in camera factories, and should be part of the equipment of base laboratories where repairing and overhauling of cameras is done. =Lens Mounts.=—All that is required for the mounting of an aerial camera lens is a rigid platform, with provision for enough motion of the lens to adjust its focus accurately. As already explained, the lens works at fixed, infinity, focus, and therefore needs no adjustment during use. It must be held far more rigidly than would be possible by the bellows, which is an almost invariable adjunct of focussing cameras. The use of ordinary types of hand cameras on a plane is rarely successful just because of the bellows, which is strained and rattled by the rush of wind. The lens mountings thus far used have been simple affairs. In the French cameras the lens is merely screwed into a flange which in turn is fastened by screws to a platform in the camera body. Adjustment for focussing is not provided; instead, the flange is raised on thin metal rings or washers, cut of such thickness by trial as to bring the lens to focus, once and for all.