AERIAL IMAGE (Fr., Image aerienne ; Ger., A etherisch Bad) A properly corrected lens produces, as it were, an aerial model at its focus of the scene at which it is directed. Each portion of this model is at the same relative distance from other portions as are the corresponding parts in the scene itself. Different parts of this aerial model are brought into focus on the ground-glass screen as the lens is racked in or out, so that the pic ture shown on the screen and recorded by the plate may be regarded as a section, vertical to the axis of the lens, through the many light rays which constitute, or proceed from, the aerial model. Owing to the coarse grain of the ground glass screens at first supplied with photographic apparatus, it used to be thought that critical focusing could only be accomplished when the aerial image is directly inspected, which could be done by means of a telescope attached to the camera and arranged to focus simultaneously with it, or by having a transparent spot on the focusing screen, made by cementing a micro scopic cover glass on the latter with Canada balsam. This idea is theoretically correct for
obtaining the greatest possible sharpness ; but general satisfaction is now given by the definition obtainable by the ordinary method of focusing, especially if finely-ground glass is used. Ground glass of quite superior fineness is now procurable which, in conjunction with a really good lens, should remove most of the difficulties met with by earlier workers. An advantage of the tele scope attachment was that it enabled moving objects to be followed, with the plate in position ready for exposure, a result now achieved more conveniently by the use of a reflex camera.