on the ground-glass of the camera, but on a specially prepared sheet of obscured glass inserted in the dark slide, which is slid into the grooves at the back of the camera. The specially pre-pared sheet of obscured glass was made by slightly etching the surface of a perfectly true piece of thin plate-glass by means of hydrofluoric acid, and this sheet, when looked through, shows merely a very thin milky haze, which shows no grain, the centre being marked by a cross in fine Indian ink. Focussing is easily effected by means of a magnifier, and the centre of the screen is made to coincide with the central division of the inch tape. The dark slide is now removed, a slow dry plate inserted in the dark-room, an exposure made, the plate developed, and the resulting negative, when dry, carefully examined by the magnifier. The image of the inch tape at once affords a guide as to the limit of sharp focus, and it is therefore possible to state without trouble the extent of sharp focus expressed in terms of the focal length as suggested by Mr. Debenham. For example, a lens was received for examination which gave the following data : -
This lens would therefore cover any plate sharply, the diagonal of which was not more than In the case of small lenses,
or lenses of short focus, a still more practical test may be pre-ferred, and that is to focus sharply, as described above, on the bricks of the house opposite one's first-floor window, throwing the window open.
(4) To Determine the Flatness of Field. The foregoing tests aie quite sufficient to determine this, but if an actual illustration is required of the amount of the curvature of the field we have only to use the newspaper again, and by marking on the base-board the focus for the centre, and then refocussing for the margins, and again marking, we can easily plot out a curve which shall show us exactly the form of the field of the lens.
(5) The Freedom from Spherical and Chromatic Aberrations. The tests for spherical aberration are comparatively easy ones. A steady, naked flame, such as a candle or small gas flame, may