276 LIGHT AND ITS ARTIFICIAL PRODUCTION. 



cleaned by washing, etc, are collected in large receivers and maintained 

 under pressure, by means of which they are distributed to consumers 

 through iron pipes. Illuminating gas, or "philosophical light," as one 

 of the first producers called it in his ecstatic joy, is therefore coal gas. 

 That which remains of the coal in the retort Is coke. 



Looked upon at first as a wonder, illuminating gas seemed destined 

 to entirely supplant tallow and oil. But it was not thus. It rather 

 formed an incentive to improve the existing sources of illumination so 

 that they could compete with it. 



In the same manner the introduction of the electric light has not 

 been a deathblow to gas, but has given it brighter life in the shape of 

 Auer's incandescent gas light, and, as the discovery of acetylene shows, 

 there are still further successes possible. 



Every system of illumination has its individual peculiarities and 

 especial advantages, which justify its existence and its worth. For 

 this reason it is not easy to estimate the relative values of the various 

 kinds of illumination unless the comparison be made on the rather 

 unsatisfactory basis of the price per candlepower; that is, by making 

 photometry the supreme judge. 



MEASUREMENT OF CANDLEPOWER. 



It is a comparatively simple matter to determine by photometry the 

 relative intensity of the light emitted by two sources. All it requires 



S is a piece of paper 



with a grease spot on 

 it and a graduated 



_-^_/ 9 scale along which the 



E sources to be com- 

 pared can be moved. 

 [Experiment.] If 

 the source E (fig. 1) 

 is alone acting, the 

 grease spot F on the 

 A paper screen SS ap- 



pears to the eye at 

 FiG.i. A dark on a bright 



background, since less light is reflected from the greased paper than 

 from the ungreased portion of the screen. On the other hand, if the 

 source L alone is active, the grease spot appears bright on a dark 

 background. If both sources are active at the same time the effect 

 will be similar to the above cases according to whether B. sends more 

 or less light, respectively, to the screen than L.^ 



The luminous intensity produced at the screen depends, firstly, on 

 the intensity of the source or its candlepower, and secondly, on the 

 distance of the source from the screen, so that the intensity diminishes 



iThis is on the assumption that the screen and the grease spot on it satisfy certain 

 conditions. For tliese and for the bibliography see O. Lummer and E. Brodhun: 

 Photometrische Untersuchnngen, Zeitschrift fiir Instrumenten-Kunde, 1889. 



