720. As leaders in the manufacture of photographic lenses and shutters, we mention the well-known name of the Bausch & Lomb Optical Company, whose extensive factory-is located at Rochester, N. Y. This company produces many lenses after their own formulae, but they are also the sole American manufacturers of the world-famous Carl Zeiss Lenses, including Planar, Unar, Tessar and Protar. These are produced under contract which the Bausch & Lomb Optical Company has as the sole American licensee, under identically the same formulae, methods of production and critical tests in process and after completion. All changes and improvements are immediately imported and carried out by these people. The following detailed descriptions of Bausch & Lomb lenses have been carefully prepared and contributed by the Bausch & Lomb Optical Company:

721. Our aim is the production of perfect lenses. To this end we are constantly introducing new and improved machinery and methods. Each element that enters into the manufacture of our products, as well as the completed article, is subjected to the most accurate and exhaustive tests, to demonstrate its optical and mechanical construction before it is allowed to leave the factory.

722. The Bausch & Lomb-Zeiss Tessar is an unsymmet-rical doublet of four very thin lenses (See Illustration No. 52) made of a new type of Jena glass manufactured especially for this purpose. The front lens is uncemented, the two elements being divided by very appreciable air space. The rear lens is cemented. The front and rear combinations are separated sufficiently to allow the Volute or Iris Diaphragm Shutters to be properly fitted. Tessar construction is essentially characterized by simplicity.

Bausch Lomb Lenses 060092

Illustration No. 52

723. The Tessar lens is light and compact, and therefore particularly suitable for use with the hand camera. The actual size of the 4x5 Tessar is shown in Illustration 53.

724. Optical Qualities

Optical Qualities. Definition - Here the Tessar lens stands supreme. The image possesses a sharpness and crispness not previously thought of in connection with such great aperture and rapidity. The corrections are perfect, the accuracy and precision of the image extending over a field of great angular area.

725. Illumination

Illumination. Because of the small number of lenses, Tessar gathers a marvelously strong beam of light and transmits it with undiminished intensity. The illumination is characterized by unusual uniformity from center to margin of plate.

726. Rapidity

Rapidity. Because of this unusual light-gathering power a rapidity indicated by f.6.3 is attained. This is sufficient for all kinds of instantaneous photography.

727. A lens combining such simplicity of construction and such speed with equally uniform illumination and precise definition has never before been placed on the market.