170 COUNT RUMFORD. 



known photometer. Placing two lights in front of a 

 white screen, and at the same distance from it, and 

 fixing an opaque rod between the lights and the screen, 

 he obtained two shadows corresponding to the two lights. 

 When the lights were equally intense, the shadows 

 were equally dark, but when one of the lights was more 

 powerful than the other, the shadow corresponding to 

 that other was rendered pale, because the light from the 

 most intense source fell upon it. Eemoving the more 

 intense light farther from the screen, until a point 

 was reached when the shadows appeared equal, Eum- 

 ford obtained all the elements necessary for the com- 

 putation of the relative intensities of the lights. He 

 had only to apply the law of inverse squares, which 

 makes a double distance correspond to a fourfold in- 

 tensity, a treble distance to a ninefold intensity, and so 

 on. In connection with these experiments he dwells 

 repeatedly upon a defect which harasses the official gas- 

 examiners of the present day, and that is, the fluctua- 

 tions of the candles used as standards of measurement. 

 These photometric measurements are succeeded by a 

 brief but beautiful essay on " Coloured Shadows," which, 

 in connection with another short essay on the "Harmony 

 of Colours," strikingly illustrates Eumford's penetration 

 and experimental skill. He produced two shadows, one 

 from daylight, the other from candle-light. The day- 

 light shadow being shone upon by the candle, was, as 

 might be expected, yellow, because the candle sheds a 

 yellow light. But the other shadow, instead of being 

 colourless, was " the most beautiful blue that it was pos- 

 sible to imagine." He states clearly that the colour of 

 one shadow is real, while that of the other is imaginary. 

 He finds it " impossible to produce two shadows at the 

 same time from the same body, the one answering to a 

 beam of daylight, and the other to the light of a candle 



