chemical focus, and we at once determine whether this is beyond or within the visual focus. (See also Focimeter.)
(6) Freedom from Flare and Ghosts. Flare is visible as a central patch of light, and arises from an incorrect position of the diaphragm in single lenses, and from reflections from the surfaces of the lenses in doublets. To test for the same, the plan sug-gested by Mr. Debenham may be adopted. A sheet of black velvet or deep red cloth is hung up in a room at night, and in front of it, at some little distance, eighteen to twenty-four inches, is placed a lighted candle. The image of the flame is sharply focussed, and there will be generally seen a halo or ring of dif-fused light surrounding the flame. The camera is now slightly slewed round, and the image watched; if the ring or halo of diffused light is steady at the centre of the screen, it may be assumed to be "flare"; if, on the other hand, the halo moves with the image of the flame, it is due to a secondary image or ghost very much out of focus. To actually determine this, rack the camera in or out to about half or double the focal length, when a small image of the flame will be seen surrounded by a large disc of light, the small image being the previous flare, and the halo the real image formed at the focus. " Ghosts " are the images of a brightly lit object reflected by the surfaces of the lens on to another portion of the screen. These may be tested for as described for flare only ; the image of the candle flame should be brought to one side of the screen, and the ghosts looked for on the opposite side. One or more ghosts may make their appearance, and will be easily visible in this way. Practi-cally, " ghosts " are a source of trouble when photographing a dark interior which contains a brilliantly lit window, as a ghost of the window may make its appearance on the opposite side of the plate in a dark portion, and in portraiture a white shirt front may appear duplicated.
(7) Accurate Centring. To test a lens for accurate centring, it is advisable to mount the lens temporarily in the camera and focus on a naked candle or gas flame placed a little to one side, then turn back the focussing screen, and several images of the flame will be seen on looking into the camera. If the lens is correctly centred, these images remain stationary when the lens is unscrewed ; if they do not, the lens should be returned to the'maker.
(8) Perfect Polishing of the Surfaces. This is by no means