If we can get nothing but a blurred image of a point, wherever we put the plate, then spherical aberration, or some phase of it, must exist. In a direct pencil this aberration can exist only in its simplest form, the effect of which is a circular blur. This may be a disc, or an ill-defined bright spot surrounded by a feeble blur, or a bright ring around a comparatively dark space, or a similar appearance with the addition of a bright spot in the centre. These different effects appear as the screen is moved to or from the lens, and the bright spot with surrounding blur is the best focus attainable. The ring with dark centre is most characteristic, and if it is found nearer the lens than the best focus the aberration is under-corrected and styled " positive," while if the ring is beyond the focus the aberration is over-corrected and " negative." Any cheap form of photographic lens, if removed from its mount, will show these effects on a fairly large scale.