TESTING ILLUMINATING GAS. 57 



cisely equal, and then, by observing the distances at which 

 the two lights are placed, and squaring the numbers repre- 

 senting them, we get their relative intensities. 



Another instrument still, which helps to show how many 

 methods have been devised to accomplish this purpose, is 

 known as Ritchie's Photometer. 



BITCniE'8 PI1OTOM 



The engraving shows it in section. It consists of a box 

 (a 5), with openings on the opposite sides for the admission 

 of light from the two sources that are to be compared. In 

 the centre, above, is a conical tube, open at the top at d. 

 Here the eye of the observer is to be placed to compare 

 the effects of the two lights, which shine upon two slopes 

 of white paper, e/and e g, which come together at e. One 

 light or the other is to be moved until the degree of il- 

 lumination produced by them upon the paper is the same. 

 The intensity of the radiance, then, from the two sources 

 will be in proportion to the squares of their distances from 

 the centre of the box. 



Instruments constructed on these principles, but quite 

 complicated in their details, are fitted up in gas-works to 

 determine the quality of the gas. In France the intensity 

 of the light is estimated by comparing it with that fur- 

 nished by a certain amount and quality of oil burning at 

 a certain rate per hour, and in England the standard of 

 C2 



