LIGHT AND ITS ARTIFICIAL PRODUCTION. 



277 



to one fourth, one-ninth, one- sixteenth, etc., when the distance is 

 doubled, trebled, quadrupled, etc. By regulating the distance R F and 

 L F the grease spot then can be made to disappear as well as by vary- 



EF^ 



ing the intensity of the sources. In this case the ratio gives 



directly the ratio of the candlepower of L to that of R. (Compare 

 note on page 276.) 



In Germany the luminous intensities of all sources are expressed in 

 terms of that of a Hefner- Alteneck lamp; e. g., gaslight is said to 

 have an intensity of a certain number of the above units. Measure- 

 ments of this kind are made in the Physikalisch-Technischen Eeichsan- 

 stalt in Charlottenburg, and are officially certified to. Instead of the 

 Bunsen photometer, above described, a more exact and more easily 

 manipulated photometer is used. This was designed by E. Brodhun 

 and myself and was constructed according to our specifications by Fr. 

 Schmidt & Haensch, Berlin. It differs from Bunseu's principally in 

 that the grease spot is replaced by a purely optical device by means of 

 which the "ideal grease spot" is realized, one which transmits all the 

 incident light and reflects none. [Experiment.] 



By means of this device, and on account of the better definition of 

 the ideal grease spot in comparison with Bunsen's, the accuracy of pho- 

 tometric measurements is much increased. I can not dwell on this 

 point, but let it suffice to say that by means of the Lummer-Brodhun 

 photometer the intensity of a source can with ease be determined to 

 one fourth per cent of its value. 



SOURCES OF ILLUMINATION ARRANGED ON A PHOTOMETRIC BASIS 

 IN ORDER OF THEIR RELATIVE COST. 



If we know in addition to the intensity of a light its price per hour 

 of use, we have a measure of its relative economic value. 



In the following table you will find interesting data concerning the 

 price per unit intensity (1 unit = 1 Hefner lamp) per hour of the vari- 

 ous kinds of illumination most used at present, on the basis of the 

 assumptions made, as stated in the table. 



Note.— The table given by the author has been converted into customary units by the translator. 

 One Hefner unit equals approximately one caudle power. 



