an easy test for an amateur, but it is possible to partly judge by examining the surfaces with a very powerful eye-piece, or by remembering that a highly polished! surface is invisible.
(9) Freedom from Stria, or Waves. The presence or absence of these defects may be observed by the method of holding the lens up to a flame, as already described.
Adjusting the Focal Length of a Lens by Alteri?ig the Relative Distance of the Constituents. The above principle is involved in the ordinary use of the telephoto lens, but it is only recently that much attention has been given to the subject in relation to ordinary photographic work. Mr. J. A. Hodges, writing in The Amateur Photographer for September 20th, 1901, p. 220, points out that by unscrewing the front element of the Cooke lens a little way so as to increase the distance between the front element and the central element, the focal length may be considerably shortened; indeed, in the special case mentioned by Mr. Hodges, the focal length was reduced from Obviously, then, a little unscrewing of
the front element of such a lens may be resorted to as a means of focussing when near objects are to be photographed with a rigid or set camera; this course being often more convenient than the use of a supplementary lens or so-called magnifier. Two recent patents embodying applications of the same prin-ciple may be mentioned: Taylor, No. 6029, a.d. 1900, and Adams & Watson, No. 8099, a.d. 1900. The matter is further dealt with at some length in The Amateur Photographer, December 19th, 1901, p 490. Opticians will understand that the essence of such a shortening of the focal length is the in-creased separation of a positive and negative element, and not the increased separation of two positive elements; indeed, this latter would have a contrary effect.
In connection with lenses, the following, among other articles, may be consulted: Aberrations, Adon, Balsaming, Focal Length, Spherical Aberration, Amplifier, Chromatic Aber-ration, Compensator, Monocle, Pinhole Photography, Curvature of Field, and Light.
Leptographic Paper (Xctttos, delicate). A collodio-chloride paper sold in France in the early days of collodio chloride (about 1866).