American cinematographer (Oct 1933)

210 American Cinematographer • October 1933 Series II. Fig. 1. Three Lens Systems, Aperature f 1 -3.5 N EARLY every modern productive industry depends upon the use of machinery or tools which embody principles of mechanical science. In this sense almost all industry is based upon mechanical engineering; and that is why the knowledge and interests of the members of the Institution, who are the tool-makers for all industry, are so exceptionally varied. Thinking that it would interest you more than would any other subject within my knowledge, I shall speak in this Address on the application of mechanical engineering to the production of lenses, particularly photographic lenses. One of the characteristics of that industry is the extra- ordinary accuracy regularly attained in its best products. One-thousandth of an inch is approximately the limit of accuracy which can be attained in the ordinary machining of metal with cutting tools. One ten-thousandth of an inch represents the order of accuracy generally attainable in such work by grinding or lapping; for example, in the work of the fine tool-maker and the watchmaker. Here are two knife-edge gauges, made to demonstrate to the King and Queen these dimensions of one-thousandth and one ten- thousandth of an inch. Each comprises a hardened and lapped straight-edge and a piece of steel, hardened and lapped to a plane and placed over the knife-edge, so that light can be seen through the gap between them. The one gauge has a gap of 0.001 inch and the other of 0.0001 inch. The latter is the highest order of accuracy attained regularly in any manufacturing industry excepting perhaps the optical industry alone. But in making the best photographic lenses and other optical instruments of precision, the accuracy of the surfaces of the elements (such as lenses .prisms, and mirrors) must be from 0.00001 to a few millionths of an inch (measured in wavelengths of light) ; and this accuracy is attained in every- day working not only by skilled artist craftsmen of long experience but by less skilled persons doing repetition work by the aid of special appliances, the products of mechanical engineering. When I entered the optical industry some forty-five years ago, after training as an engineer, I was greatly im- pressed by the primitive methods then in use and the won- derful work done by a few artist craftsmen. Alvan Clarke, who constructed the great telescope objectives of the Lick and Yerkes Observatories, appeared to have no tools much more elaborate than those one could find in a country blacksmith’s shop. But the product of the artist craftsman *—Reprint of address delivered to The Institution of Mechanical Engineers. Mechanics is expensive and its quality inevitably varies with the crafts- man. It may or may not be really good, and the quality is rarely upheld and ensured by complete and independent in- spection. These things were true, for example, of the old English hand-made watch. Before describing how the production of lenses, such as those used in photography, has been lifted from the level of a somewhat primitive craft towards that of a science, and put, as Sir Charles Parsons expressed it, upon a mech- anical engineering basis, I must briefly describe in a very general way the characteristics of such lenses. The Designing of Photographic Lenses. —The function of a photographic lens is to form real images of objects by receiving light from each point of the object, and con- densing it to a corresponding fine point properly located in the image. The problem of satisfying this simple statement in de- signing the best photographic lenses is exceedingly complex. No true mathematical solution is known nor perhaps ever will be. The only really perfect optical instrument is the plane mirror. The work of photographic lens design is Fig. 5. Croup of Finished Lenses. At bot- tom Fig. 7, Typical pieces of raw glass plate.