quently the silk alone is used as a reflecting surface. Much flexibility can be secured as the silk may be left highly transluent or diffusing and additional layers may be added to secure the desired transmission and different colors for different effects.

In an effort to make semi-indirect units more universal and allow them to be used even where highly reflecting ceilings are not available or in those locations where the ceilings are too high above the logical locations of the unit the fixture manufacturer has designed a small portion of ceiling to go with the bowl. The fixture therefore consists of a diffusing bowl and a reflecting surface a short distance above. This method serves to enlarge slightly the light giving area, and thus to decrease correspondingly the fixture brightness. The upper reflecting surface has been changed in size, location and shape by the several manufacturers, but always serves the same purpose.

A survey of the field indicates that excellent accessories of a wide variety are available. As a rule, however, these follow conventional lines according to well recognized concepts of design and use. Only to a slight extent are designers undertaking to provide accessories which represent adaptation of simple means of directing, diffusing and tinting the light along unconventional lines.

References

E. B. RowE. "Some tendencies in the design of illuminating glassware." Electrical Engineering, Sepember, 1914.

JAMES R. CRAVATH.-"Glass globes for street lamps." Municipal Journal, August 27, 1914.

RENE CHASSERIAUD.-"The art of logical lighting (French)." Societe Belges des Electriciens (Brussells), May, 1914.

GUIDO PERI.-"Present status and tendencies in electric illumination (Italian)." L'Industria (Milan), November 1, 1914.

H. B. WHEELER.-"Lighting of show windows." Illuminating Engineering Society Transactions, September, 1913.

W. W. COBLENTZ.—“The diffuse reflecting power of various substances." Bulletin of Bureau of Standards, April 1, 1913.

H. J. TAITE and T. W. ROLPH.-"Notes on metal reflector design." General Electric Review, May, 1914.

A. L. POWELL.-"An investigation of reflectors for tungsten lamps." General Electric Review, November, 1912.

M. LUCKIESH.-"Investigation of diffusing glassware." Electrical World, November 16, 1912.

W. W. COBLENTZ.-"Diffuse reflecting power of various substances." Journal Franklin Institute, November, 1912.

L. BLOCH.-"Reflectors and accessories for lighting inner rooms with metal

filament lamps (German)." Elektrotechnik Und Maschinenbau, October

13, 1912.

DR. L. BLOCH.-"Reflectors for metal-filament lamps." London Electrician, March 21, 1913.

A. L. POWELL and G. H. STICKNEY.-"Data concerning incandescent reflectors." Electrical World, September 6, 1913.

VAN RENSSELAER LANSINGH.-"Characteristics of enclosing glassware." Illuminating Engineering Society Transactions, September, 1913.

W. T. MACCALL.-"Half frosted lamps in reflectors." London Electrician, October 3, 1913.

A. L. POWELL.-"Reflectors for tungsten lamps in industrial and office lighting." Electrical Engineering, October, 1913.

DR. L. BLOCH.-" Choice of reflectors for street lighting." London Electrician, May 31, 1912.

L. BLOCH.-" Choice of reflectors and proper heights for metal filament street lamps." Elektrotechnik und Maschinenbau, December 3, 1911.

R. HORATIO WRIGHT.-"The mazda lamps with a few common types of reflectors." Sibley Journal of Engineering, November, 1910.

C. TOONE. "Globes, shades and reflectors." London Electrical Review, June 16, 1911.

P. G. NUTTING, L. A. JONES and F. A. ELLIOTT.-"Tests of some possible reflecting power standards.” Illuminating Engineering Society Transactions, Volume 9, No. 7, 1914.

LEONARD MURPHY and H. L. MORGAN.-"Distribution and efficiency tests on lamp shades and reflectors." London Electrical Review, July 7, 1911.

GEO. H. MCCORMACK, ALBERT JACKSON MARSHALL, L. W. YOUNG; Introductory remarks by BASSETT JONES, JR.-"Symposium on illuminating glassware." Illuminating Engineering Society Transactions, September, 1911. THOMAS W. ROLPH.-"Reflectors for incandescent lamps." Electric Journal, May, 1910.

J. R. CRAVATH.-"Show window illumination." Central Stations, May

1910.

C. E. FERREE and G. RAND.-"Some experiments on the eye with inverted ieflectors of different densities." Illuminating Engineering Society Transactrons, December 20, 1915.

FRANK A. BENFORD.-"The parabolic mirror." Illuminating Engineering Society Transactions, December 20, 1915.

HAYDEN T. HARRISON.-"Efficiency of projectors and reflectors." Abstract of a paper read before the Liverpool Engineering Society.

LIGHT PROJECTION: ITS APPLICATIONS

BY E. J. EDWARDS AND H. H. MAGDSICK

Light projection, as the term is commonly employed, covers the redirection of light flux from artificial sources by means of suitable optical systems so that it may be utilized within solid angles which are small as compared with those encountered in equipment for general illumination purposes. It was in connection with such applications in a few restricted fields that some of the more important principles of optics and illuminating engineering were long since developed and applied. During the past few years these applications have multiplied rapidly, occupying the attention of many illuminating engineers and giving rise to numerous papers in the Transactions of the Illuminating Engineering Society and articles in the technical press dealing with the principles of optics, searchlighting for military and navigation purposes, flood-light projectors for displaying surfaces at a distance, headlighting for vehicles, orientation lighting for the navigator, light signals, and apparatus for the projection of enlargements of transparencies.

Two general classes of apparatus are used to direct the flux from a source into the desired small angle: Opaque reflector systems controlling the light by the principle of specular reflection, and lens systems depending upon the refractive properties of glass. Frequently the two forms of control are combined in the same device.

In Fig. 1, A, is illustrated the action of a simple convex lens. A light ray emerging from the focus, F, is refracted in passing through the lens so as to be projected parallel with the axis, while from a larger source as shown at the focus, a cone of light is projected with an angle of divergence, 2b, depending upon the size of the source, the focal length of the lens and the angle, a, at which it is emitted. The greatest angle of divergence is that of the cone issuing at the axis of the lens. These statements apply to lenses intercepting the flux in a relatively small solid angle. As the diameter of a lens increases relative to the focal length, the thickness, and hence the absorption, increase rapidly and the control of the emerging rays is limited by the increasing spherical and chromatic aberration. To

reduce these elements of inefficiency, Fresnel nearly one hundred years ago built a lens of concentric rings, Fig. 1, B; in effect a large convex lens with sections of the glass removed. He also added concentric prism rings to direct additional light into the beam by total reflection. Later these prisms were given a curved surface and re

fraction was combined with reflection to produce the desired results. It will be noted that the sections give rise to a series of dark rings when viewed within the beam, since the light striking the risers is deflected at a large angle from the axis. In Fresnel lenses of reason

Axis

Axis

B

Fig. 2.-Light projection with opaque reflectors.

ably effective angle, the solid angle subtended by the lens at the focus, the contour of the surface may be so corrected as to secure very accurate control of light. They are frequently referred to as stepped or as corrugated lenses.

Rays emerging from a source at the center of from the polished surface as shown in Fig. 2, A.

a sphere are reflected Used in this manner