The results of the exploration of the field of light around a given source are usually ex pressed by means of curves. The curve showing the distribution in a horizontal plane is called the curve of horizontal intensities. The cor responding curve for any vertical plane is a curve of vertical intensities. In Fig. 9 are given two typical curves of vertical intensities from measurements of an open-arc lamp without shade. Fig. 10 is a diagram showing the•llumination of the ground at different angles when such a lamp is used in street lighting. When the curve of horizontal intensities is a circle, a single curve of vertical intensities suffices to completely de scribe the performance of the source of light ; when, however, as is usually the case, the curve of horizontal intensities is not a circle, the curve of vertical intensities will vary in form as the vertical plane to which the curve applies is revolved about a vertical axis through the cen tre of the source of light. The average inten sity of a source of light viewed in the horizontal plane is called the mean horizontal intensity. The average intensity, taking into consideration every possible point of view, is called the mean spherical intensity of the source.
The rigorous determination of either of these quantities in the case of unsymmetrical sources would take an infinite number of readings. In the case of the incandescent lamp, which will permit of such treatment, it is, however, pos sible to obtain the mean horizontal intensity by rotating the lamp upon its vertical axis at the end of the photometer bar. Measurements made upon the lamp revolving thus at a speed of several revolutions per second are found to agree well with the mean horizontal intensity as determined by plotting a series of readings taken from different positions at a horizontal plane with the lamp at rest, and integrating the curve thus obtained. The approximate deter mination of mean spherical intensities of non symmetrical sources such as the arc lamp, when made with the ordinary photometer and mirror, involves the taking of many individual read ings. In the Franklin Institute tests, made at the Electrical Exhibition in Philadelphia in 1885, 65 measurements were made in each determina tion and these were combined in computing the main spherical intensity. These readings
were made from 38 directions distributed as evenly as possible around a sphere of which the light was the centre. To avoid this labori ous process numerous special forms of photom eter have been devised of which the most successful is that invented by Matthews and used in the extensive studies of the arc light recently carried on under the auspices of the National Electric Light Association. In this instrument which is known as Matthews' inte grating photometer 24 large mirrors are ar ranged around the source, the mean spherical intensity of which is to be determined, in such a way as to produce upon the photometer screen an illumination proportional to the mean spherical intensity. The adjustment of these mirrors is such as to direct simultaneously to ward the photometer the beams of light which the eye of an observer would receive if he were to view the source successively at angular intervals of 15 degrees in a vertical plane. The intensity of the light received in these various directions must further be reduced in the ratio of the sine of the angle between the direction of view and the vertical, and this is .done by the interposition of glasses which are smoked until they transmit the desired proportion of the light falling upon them. With this instru ment the photometer gives by a single reading a value proportional to the mean spherical in tensity of the source.
For the application of photometry to astron omy, see STARS. For the application of the Selenium cell of photometry, see SELENIUM.
Bibliography.— Palaz, 'La Photometric In dustrielle) (translation by Patterson) ; Stine, 'Photometrical ; Bell, 'The Art of Consult also the reports of the committee on photometry in the 'Proceedings' of the National Electric Light Association; various papers in the of the American Institute of Electrical Engineers, in 'La Lumiere Electrique,' in the nische Zeitschrift,' and in Schilling's Journal fiir Gas- and Wasserversorgung.