rotated slightly so that the lenses and prisms appear in half-shadow. Film and poor polish then appear as a slight haze over the surfaces, which with practice can be detected at a glance. During the war much trouble was encountered because of the appearance of film on lens surfaces; the source of trouble was found to be different in different cases. In some instruments unstable glass caused the trouble; in others the film was found to be organic in nature and deposited from volatile matter such as oil, grease, poorly baked lacquer, etc., included in the instrument at the time of its assembly and adjustment. The general conclusion, reached as a result of many researches into the causes of film, has been that only weather-resistant glass of good quality should be used in military optical instruments and that the most painstaking care should be taken in the assembly of optical instruments to insure cleanliness and freedom from grease and volatile matter in the interior of telescope tubes; under no conditions should the operator's fingers touch the lenses and prisms, after cleaning, during their assembly into the instrument. Failure to observe these simple precautions and to provide proper assembly rooms free from dust, was the cause of many rejections of optical instruments, especially binoculars, during the war.

The light-transmission of a telescope depends on a number of factors, such as quality and kinds of glass, total glass path, number of reflecting surfaces, quality of polish of the surfaces, condition of Canada balsam layer in cemented lenses. There are available different methods for measuring the light-transmission of telescopes; several of these were described in Chapter IV and need not be repeated here. They are without exception photometric in nature. The essential difference between determining the light-transmission of a piece of glass and a telescope is that in the telescope the rays follow prescribed paths whereas in the glass plate they may be transmitted along any direction.

7

For the measurement of the transmission of a telescope it is essential that the light rays follow a telecentric course and that the exit pupil of the telescope be imaged in the field of the photometer. A simple attachment to the Koenig-Martens polarization photometer is shown in figure 71, page 214. It consists essentially of an achromatic lens mounted in a brass cylinder which slips over the front tube of the polarization photometer and images the exit pupil or entrance pupil of the telescope in the field of the photometer. The lens is so placed that the front aperture of the photometer coincides with its rear focal plane. Either the objective or eyepiece end of the telescope may face the photometer. The source of illumination is the same as that described in Chapter IV; also the method 7 F. E. Wright, Jour. Opt. Soc. America, II-III, 65, 1919.

39229-21-19

of measurement in which the position angles of the analyzer in the photometer are recorded for settings with the telescope in the field and for settings without the telescope. The ratio of the squares of the tangents of the angles thus obtained is a measure of the percentage light transmission of the telescope.

A photometric bench may also be used to advantage for measuring the percentage light transmission of telescopes. An instrument of this type was manufactured during the war by Keuffel and Esser (Fig. 69, page 212) for the use of inspectors of optical instruments and proved to be satisfactory in practical work. Another type of bench photometer for the measurement of the light transmission of optical glass and of optical instruments is that described by C. V. Drysdale.s

8 Trans. Opt. Soc. London, p. 100, 1902; 18, 375, 1917.

Chapter VII.

THE OPTICAL INSTRUMENT SITUATION DURING THE

WAR.

In the foregoing chapters a general description is given of the processes of manufacture of optical glass and of the optical parts of lens systems. Emphasis is placed on those processes which were developed during the war period and proved to be suitable for use in an emergency. No picture is, however, given of the development of the optical situation, as a whole, and of the measures taken to meet the ever-increasing demands of the Army and Navy for military optical instruments. As a matter of record a brief sketch is presented in the present chapter of the progress made and of difficulties overcome. In retrospect and with the facts before us it is now a simple matter to state how this and that should have been done; but at the time decisions had to be made on the fragmentary evidence available. The records show, that, although there was much waste effort and confusion, the Army and Navy were actually supplied with most of the optical instruments which they needed; also that Army and Navy at maximum strength in 1919 would have been adequately equipped with firecontrol and other optical instruments.

The development of the optical-glass situation is briefly described in Chapter I. Several details may be added here to illustrate the kinds of problems which arose and the manner in which they were solved. These included problems of factory organization, of factory operation, of the procurement and transportation of raw materials and of glass melting pots and of coal; also the more difficult task of obtaining hearty cooperation from certain manufacturers who were vitally interested, but who extended formal cooperation only with a strong undercurrent of passive resistance. Cooperation of this kind leads to innumerable delays and unfilled promises and must be treated both tactfully and firmly to accomplish the desired results.

In the game of war-time production everything is subordinated to the one object of producing the desired material within a definite period; the military program requires munitions of many different kinds, and it is the task of the manufacturing forces of the country to furnish these on time. Everything else, including expense, is for the moment, subordinated to speed; as the manufacturing program

gathers momentum, the half-hearted cooperationists either learn their lesson or they are eliminated. Under the stress of high-speed production suggestions and plans for increasing the rate of production are made; and if, after adequate test, these prove to be satisfactory, they are introduced into the factory routine. Many of these suggestions come from outside plants.

Under the emergency conditions many manufacturers of broad vision are willing to exchange information on factory practice with their peace-time competitors with the result that over the entire country the manufacture of munitions soon attains a state of efficient operations.

The procurement and the transportation of the raw materials for optical glass required constant attention throughout the war. At first it was necessary to locate satisfactory sources of supply for these materials. Specifications had to be stated with reference both to the optical glass requirements and to the chemical manufacturing possibilities. Many chemical manufacturing plants were visited by members of the Geophysical Laboratory before the details of the supply of raw materials were properly arranged. The problem of transportation continued to be a constant source of trouble throughout the war period. Innumerable delays in the shipment of raw materials occurred; in many instances the General Munitions Board, and later the War Industries Board and the Production Division of the Ordnance Department rendered valuable service in expediting railway traffic. It is inevitable, however, that in a war-time emergency railway traffic is overtaxed. Under these conditions it is the duty of each manufacturer of munitions to stock, as early as possible, adequate quantities of supplies so that the inevitable delays in the transportation of additional supplies do not retard production. At one time during the early months of the war about 40 manufacturers were actively assisting in the manufacture of optical glass, chiefly in the supply of the necessary raw materials and of glass melting pots.

The question of fuel and gas for the glass-melting furnaces and for other operations became serious during the coal shortage of the winter 1917-18. When it is realized that the glass plant at the Bausch & Lomb factory alone consumed 33,000,000 cubic feet of illuminating gas per month, a quantity sufficient to meet the needs of a city of 80,000 inhabitants, the scale of its fuel consumption and of the difficulty of meeting the situation adequately is apparent.

From April to December, 1917, the efforts of the Geophysical Laboratory were concentrated chiefly on the development of the manufacture of optical glass. By December, 1917, the production of pot optical glass had reached 40 tons per month; at one plant (Bausch & Lomb) the processes of manufacture had been mastered for the most part; and subsequent efforts were directed chiefly to an

increase in the manufacturing capacity for optical glass throughout the country. Early in December the Geophysical Laboratory took charge of the Spencer Lens plant and, as a result of hearty cooperation on the part of this firm and a modern though small plant, was able to produce satisfactory glass from the first melt on. Late in December the Geophysical Laboratory assumed practical charge of the Charleroi plant of the Pittsburgh Plate Glass Co. This plant was an old plate-glass plant in which the Pittsburgh Plate Glass Co. had installed 16 single-pot glass-melting furnaces of a blast-furnace type and had tried unsuccessfully for several years to produce a satisfactory product. At the time the Geophysical Laboratory arrived at the plant several of the problems involved to place the plant on a running basis were not at first apparent, especially the lack of temperature control in the glass-melting furnaces and the impossibility of establishing such control with the system then in operation. The entire battery of furnaces was supplied by air from a single lowpressure line fed by a blower operated on an electric circuit subject to rapid changes in voltage; in this system a change in the flow of air in one furnace affected the rate of flow of air in the remaining furnaces, with the result that violent fluctuations in the temperature of each furnace were the rule. Good optical glass can not be made under these conditions. Each furnace had to be equipped with individual blowers and many fundamental changes effected before satisfactory production could be attained. This required some months for accomplishment and proved to be a task of considerable difficulty, partly because of a lack of appreciation on the part of this company of the significance of the factors underlying the manufacture of optical glass and of the fact that optical glass is not plate glass, nor ́is a plate-glass maker necessarily a competent maker of optical glass.

In retrospect it is now evident that better progress would have been made, more glass produced, and much money saved had either a new optical glass plant been built or, for example, the plant of the Spencer Lens Co. been expanded rather than the attempt made to remodel an old plate-glass plant. Experience proved, furthermore, that in the manufacture of optical glass it is better to start with new hands than with plate or other glass makers, who are necessarily prejudiced and do not readily change their attitude of mind toward certain factory operations. The conservatism and inflexibility of the ordinary factory hand can be appreciated only through actual manufacturing experience.

It would seem to be a wise policy for the Government in time of war to concentrate its efforts on two plants. for the manufacture of optical glass rather than on three or more. In two plants properly situated adequate quantities of optical glass can be produced to meet