lens ; (4) the flatness of the field ; (5) the freedom from spherical and chromatic aberrations ; (6) the freedom from flare and ghosts ; (7) the accurate centring of the lenses ; (8) the perfect polishing of the surfaces ; (9) the freedom from striae or waves.
(1) To Determine the Equivalent Focus. Several methods are given under the heading Focal Length.
(2) To Determine the Ratio Aperture of the Diaphragms. This is described under Diaphragms (q.v.).
(3) The Covering Power of the Lens. To determine the covering power of the lens is by no means a difficult task, especially if the lens be used with a given-sized plate; that is, for the one for which it is advertised. Many lenses will, with small diaphragms, cover a much larger plate than that for which they are designed, and all lenses will cover a smaller plate. If you wish to test whether a lens will cover a given-sized plate, all that you require to do is to affix the lens to the front of a camera which has a focussing screen of the given size, and then focus till you obtain a sharp image at the centre ; it will be at once seen whether the lens will cover. But it is not only necessary that a lens should cover a given-sized plate, but most photographers require that, besides covering a plate, a lens should give good definition all over a plate, which is a totally different thing. Many a lens will cover a plate and yet give very poor definition at the margins, because either (a) the aberration for oblique pencils is not corrected, or (d) it has a very curved field. For special purposes such as hand-camera and instantaneous work it is now considered necessary that a lens shall give sharp defi-nition over the entire surface of the plate for which it is intended. It is often suggested that to test for this in a lens the best plan is to set up a sheet of newspaper and, focussing sharply, to examine the definition with a compound focusser. We prefer to make a practical test as suggested by Mr. W. E. Debenham, as follows: - Take a sheet of printed matter that is always to be had of one-sized type (nonpareil), namely, "Births, Deaths, and Marriages" column of the Standard newspaper. This column is placed slantwise, the top of the column being farthest from the plate, but each particular line runs square across it as before directed, in the middle of the column, and at such a distance that when a printed line is focussed in the centre of the field, the image is one-eighth of the original size. The width of two