Sept. 1925 I L L U M I N A T I O N I T E M S 1027

and thus no interference results from that great propor­tion of beams that could never be accurately adjusted or the adjustment of which has been only slightly affected in driving. The great value of such equipment has long been recognized, but it has not been widely adopted because of the cost and complications of the mechanical and electrical methods employed and the limitations of some of the designs from the lighting standpoint. The two-filament lamp has made it possible to accomplish the desired beam-depression most inexpensively, ad­vantageously, and positively, merely by switching from one filament to the other.

Optical Principles. The simple optical principles that are utilized to produce this result are illustrated in Fig. 3. When the light source is at the focus of a para­bolic reflector, the rays are reflected into a narrow beam which, in falling on a screen at right angles to the beam, produces the round spot of light shown at (a). If the filament is below the focus, as in (ä), the spot of light is extended upward into an oval. But if the filament is above the focus, as in (c), then the extension is downward and the level of the top of the beam re­mains unaltered. As the distance from the focus is increased, the distorted beam becomes still deeper, as[in(d).

p I G > 5 — E X C E L L E N T L I G H T DISTRIBUTION AND SIMPLEST

ADJUSTMENT R E S U L T W H E N THE REFLECTOR SURFACE OR L E N S ,

OR B O T H , ARE M O D I F I E D FOR FILAMENTS PLACED AS IN (A)

AND (D) OF F I G . 3

Thus, it may be seen that a lamp combining a filament as in (b) with a second one as in either (a), (c) or (d), makes possible a controllable beam, depressible at the will of the driver. Equipped with fluted cover glasses to spread the beams, and tilted so as to bring the top of the higher beam, from (&), to the level of the headlamps, the relative direction and form of the beams are as in­dicated in Fig. 4. If a greater angle of tilt is desired, it is necessary only to increase the displacement of the filament below the focus in (6). But if the desired angle of tilt is obtained with the filaments spaced at equal distances above and below the focus as in (b) and (c), then a lamp with filaments separated by the same dis­tance but placed with one at the focus as in (a) and the other below by the full amount of the separation, would produce too great a depression of the beam. It has not, in fact, been found commercially feasible to make lamps having a sufficiently small separation of the filaments, to limit the tilt to the desirable two to three degrees when one of the sources is at the focus in headlights consisting of a parabolic reflector and a cover glass with only ver­tical spreading flutes. The combination of (6) and (c) must then be used. It happens that with this system, the angle between the two beams is so much affected by

the range of filament position incident to variations in lamps and in socket assembly as to make necessary an additional focusing screw for vertical adjustment. The focusing and aiming of the headlights then ob­viously becomes somewhat more of a task. These same limitations apply in using the new lamp in the vertically fluted reflectors, as now found on cars.

However, much of present headlighting is done with other classes of equipment, in which reflectors with modified surfaces, or lenses with bending prisms, are employed. These equipments can be made in such a way as to give the most desirable form of controllable beam and still require no provision for vertical ad­justment of the bulb and no extra work in focusing and aiming. The reflectors, or lenses, or a combination of the two, can be designed so that the desired angle of tilt is obtained even though the socket is offset to bring one of the filaments on the axis of the reflector. Thus, Fig. 5 (a) shows how the modified contour of the re­flector directs downward that light which would form the upper part of the beam from the unmodified para­boloid of (d) in Fig. 4. At the same time the modified surface can be made to give the most desirable depth of main driving beam from the filament on the axis. In (b) of Fig. 5, bending prisms, covering the upper and lower parts of the beam, accomplish similar results. Some equipments of special merit employing these principles have been designed and submitted for ap­proval in connection with the new lamp.

Engineers Favor Depressible Beam. The spacing of 9/64 in. between the two filaments of the new lamp was determined after a thorough study of the best spacing for the various designs of equipment that are being developed or may be brought forth in the future, and the adaptability of the various types to one standard lamp. It is, of course, very important that all equip­ments be made for this one lamp to insure that, as renewals are needed, the motorist will find only the right one everywhere available. In setting the spacing, a thorough investigation was made of the most desirable angle of depression. If the tilt is made more than three degrees, the depressed beam lights the road insufficiently far ahead. On the other hand, if the angle is less than two degrees, there will be too many situations under which complete relief is not afforded the approaching driver.

NEW YORK-CHICAGO TELEPHONE CABLE

On August 11 at Swanton, Ohio, 19 miles west of Toledo, the last of 9500 splices was made in the New York-Chicago telephone cable, the world's longest telephone cable. The over-all length of the cable is 861 miles, 717 miles being supported overhead by 36,000 poles and 144 miles being underground. It is nearly twice as long as the next longest cable of 450 miles from Boston to Washington. The new cable will, it is expected, be open for service in about a month.