National Geographic : 1922 Jan
0 When light rings flash their warning Each Kodak Anastigmat is fashioned to conform to a master glass that is really another lens of equal but opposite curvature. The master glass represents the perfect optical curve and the lens must fit it exactly -exactly. The test to determine when this result has been secured is a very delicate and a very pretty operation. If, when the expert brings the two lenses into contact, vari-colored light rings appear, some slight difference in curvature must exist. Even though this difference may be infinitesimal, the light rings flash their warning. It is only when careful readjustment on the polishing machine has successfully corrected this error and the form of the light rings proves this fact, that the lens is good enough to become a Kodak Anastigmat. Do you know how a lens forms an image or why cameras have different kinds of lenses? Both of these questions, and others, you will find answered in a complete and interesting fashion in the little free booklet "About Lenses ". We should like to send you a copy. Th .K Eastman Kodak Company AN ATS Rochester, N.Y. .I"I' d.5 "Mention The Geographic-It identifies you"