OFTEN THIS department is asked for instructions for making a binocular. No such thing exists; nor would the job be very simple on a single instrument. Quite the contrary. The optics would be those of two refracting telescopes of short focal ratio; plus two Porro prism systems which erect the image, shorten the tube and, by increasing objective separation, enhance the stereoscopic effect (the dear public largely thinks they are put there to magnify); plus a mounting. In time and money, too, the job would far out cost a purchased instrument, and thus it is practically impracticable. That is just why some enterprising, nose-thumbing amateur will ultimately make a binocular. While the following résumé of recent improvements in U. S. Army binoculars, which was obtained from Frankford Arsenal, Army Ordnance Department, gives no how-to-make-it instructions, its background data should interest all readers.

THE EMERGENCY precipitated by the impending World War II presented the United States Army Ordnance Department with the problem of immediate procurement of a large quantity o binoculars. Because of the urgency for setting up production it was necessary to select the most adaptable mode for which tooling existed and to produce approximately 350,000 instruments as quickly as possible. The model selected was the 6 X 30 commercial binocular being manufactured by the Bausch and Lomb Optical Company with whom an initial order for 20,000 binoculars was placed. This binocular was given the official nomenclature "Binocular, M3," indicating Standard Army Issue model No. 3 in the "M" series of binoculars. The 6 X 30 denotes six power magnification and 30mm objective diameter.

The only change necessary in the commercial model was the incorporation of a military reticle. The reticle shown below is a glass disk, both surfaces of which are polished parallel to within two minutes of arc. On one surface lines and figures, presenting a graduated scale, are etched and filled with an opaque material to make them visible. The crossline pattern and the dimensions of the reticle are determined by the type of binocular in which the element is to be used. The element is usually made of baryta light flint or spectacle crown glass. The function of the reticle is to place the crossline pattern in the same focal plane as the real image formed by the objective so that the distance between two points or objects in the field of observation may be calculated.

Since it was apparent, even prior to placing the initial order, that the resources of the experienced Bausch and Lomb Optical Company would be necessary for the production of more critical precision optical instruments than binoculars, arrangements to set up separate facilities to produce binoculars, solely and on a large scale, were inaugurated. Nash-Kelvinator, Ranco Division, and Westinghouse Electric and Manufacturing Company were selected on the basis of Industrial Service surveys indicating their ability to utilize their existing equipment.

As neither Nash-Kelvinator nor Westinghouse had previous experience in making military optical instruments or the facilities for the manufacture of the optical elements, the Ordnance Department instituted an elaborate program for procurement of the essential optics to be furnished to the two companies for assembly with the mechanical elements into complete binoculars. Optical elements included in the binocular are Porro prisms spherical lenses, cemented doublets- that is, a crown and a flint lens cemented together-and a reticle. The complete assembly is shown below.

Fortunately, binocular optics do not require the extreme accuracy of optics such as Amici prisms and other special elements used in higher powered instruments.

Despite the fact that optics were being manufactured by approximately 50 different companies, surprisingly little difficulty through rejections and disputes between the optics processors and the instrument assemblers was experienced. This may be attributed largely to an efficient inspecting system. Inspectors especially trained at Frankford Arsenal for inspection of fire control instruments were sent out to each one of the 13 ordnance districts in the United States. Since inspection of an optical surface cannot be measured with a graduated instrument, but must be based on the judgement of an inspector, occasional controversy arose regarding acceptability. In such case, a standardization meeting was held, whereupon master inspectors from Frankford Arsenal rendered final decision.

Today's Army operates in the humid moisture-laden climate of the South Seas, in the icy climates of Greenland and Alaska, and in the African desert where night temperatures are below zero in sharp contrast to the extreme heat of the day. Binoculars used in World War II are subjected to terrific vibration and shock in transport and in action and often are completely immersed during military surf landings. From the viewpoint of World War II, Binocular M3 is a superlative instrument However, after a quantity of the M3 binoculars were issued and in use many difficulties due to the nature of military operations of World War II were reported.

As a result of these reports, a concentrated study of the most minute details of Binocular M3 was conducted, which eventually resulted in the development of Binocular M13.

The first problem to be considered in the development of the M13 from the basic M3 was that of waterproofing the instrument to withstand submersion. This was accomplished by redesigning the cover plates to provide for the use of a synthetic rubber gasket and a greater number of fastening screws. In addition, a new military wax, capable of withstanding extreme high and low temperatures, for sealing the objective lens and objective assembly was developed by the Ordnance Laboratory. This compound, Specification FXS, replaces Navy Black Sealing Compound No. 3A. It resists cracking at -50 degrees F. and has a melting point of 210 degrees F., as against 150 degrees F. for No. 3A compound. The formula includes a fungicide to repel molds and insects.

Shock and vibration tests revealed that severe shock caused shifting of the original prism mounting, affecting the optical alinement of the instrument. Experimentation with methods of mounting prisms resulted in the use of a dental cement. This cement is a blend of cupric oxide powder, phosphoric acid, and zinc chloride in solution. The ingredients are mixed in the ratio of three parts of powder to one part of liquid. Additional tests proved that prisms mounted with this agent were locked firmly against all shock remained free of strain, and could be removed readily for cleaning.

Another very serious problem, applicable to all telescopes, was the formation of moisture on the optical elements within the finished instrument. In any binocular, moisture may eventually enter and condense on the optics, because no instrument with an adjustable threaded eyepiece movement can be sealed perfectly. Such formation is most objectionable on tee graduated reticle, upon which the most trifling speck is visible and distracts the user. A plane high in the sky first appears as a tiny pinpoint which looms very like a fleck of dust under magnification of the binocular eyepiece.

Experience gained in packaging complicated items for export, using dehydrating agents, was utilized in solving this problem. A special cartridge, shown in the small illustration, containing a small amount of silica gel, was placed within the body of each binocular. The instruments so treated were tested by subjection to most adverse conditions of humidity and rapid changes in temperature, which proved that the desiccant eliminated formation of moisture on the optics over an extended period of time.

A means of making the binocular more usable under conditions of fading light was undertaken. American binoculars effective an hour later in the evening than those of the enemy would be of great advantage to American soldiers. Therefore, a development of the optical industry-coating optical surfaces with a magnesium fluoride film to reduce loss of light by reflection-had been under study by the Army and Navy for some time. By exerting the full power of research of the Army, Navy, and associated commercial facilities toward perfection of magnesium fluoride and other coating techniques, coatings were produced to withstand cleaning and all field conditions. The magnesium fluoride coating is applied to the optical surfaces at high temperature under a high vacuum, the fluoride becoming a part of the glass surface. This coating reduces light reflection and permits a greater amount of light to pass through the optical system, enabling the use of the binocular at dusk, when light is fading.

After the time and money spent in producing a fine binocular, an improvement in the export method was incorporated as further insurance that the binocular will reach the ultimate user in factory-new condition. The binocular is placed in its special leather carrying case and sealed, together with five ounces of silica gel, in a moisture-vaporproof bag and cushioned in a corrugated carton. Twenty-four such cartons are then packed in a steel-strapped wooden box having a submersion-proof bag lining.

This new M13 binocular is now mass production. Thousands of them are being shipped every month to American fighting men overseas to aid them in seeing the enemy before the enemy sees them. Of course, the Ordnance Department is never satisfied with the degree of perfection of the fighting equipment of our Army. Even now additional improvements to assure that the American binocular is the best the world are in progress.

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