PHOTOMETRY AND ELECTRIC LIGHTING. 313 



The so-called search-light presents some very interesting features in regard to con- 

 ical intensity. The numerical example given below will serve as an illustration. To 

 understand this example one must remember that light which emanates from a point 

 in the focal plane of a lens or mirror is transformed by the lens or mirror into a beam 

 of rays which are all parallel (ignoring errors of spherical and chromatic aberration and 

 astigmatism ) to a line drawn from the point to the center of the lens or to the center 

 of curvature of the mirror. Therefore the light which emanates from a small lumi- 

 nous surface in the focal plane is transformed into a series of parallel beams all the 

 rays of which lie (at a great distance from the search-light) within the cone formed 

 by drawing lines from every point of the small luminous surface to the center of the 

 lens or to the center of curvature of the mirror, and the solid angle of this cone is 

 equal to the area of the small luminous surface divided by the square of the focal 

 length of the lens (or mirror). 



The powerful arc lamp of a certain search-light emits light of 10,000 candles conical 

 intensity towards every part of a lens (or mirror) which subtends one unit of solid angle 

 as seen from the arc which is at its focus. The luminous surface of the lamp is one 

 quarter of a square centimeter and the focal length of the lens (or mirror) is 30 cen- 

 timeters. Therefore the solid angle of the cone which contains the search-light beam 

 is ^ divided by 3O 2 or yg 1 ^ of a unit, and, if we assume that no light is lost in the 

 lens (or on the mirror) by absorption, the conical intensity of the search-light beam 

 must be 3,600 times as great as the conical intensity of the light direct from the 

 lamp, or 36,000,000 candles. One who considers this example carefully will be im- 

 pressed with the important fact that the candle is not a unit of quantity of light. The 

 candle and the hefner are units of conical intensity. 



129. The Bunsen photometer is a device for measuring the 

 conical intensity of a beam of light from a given lamp in hefners 

 or in candles. It is the photometer that is almost universally 

 used in simple photometric measurements. The given lamp 

 and the standard lamp are placed at the ends of a horizontal 

 bar and a screen of thin unsized paper is moved along the bar 

 until the two sides of the screen are equally illuminated by the 

 two lamps. The intensities of illumination (sectional intensities 

 of the beams of light) due to the respective lamps are h/d* and 

 h' jd' 2 , according to equation (50), and since these are equal we 

 have : 



-? 



in which h and h' are the conical intensities in hefners or in 

 candles of the light sent towards the screen by the respective 

 lamps, and d and d! are the distances of the lamps from the screen. 



