444 Mr RITCHIE on a new Photometer, 



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tangular slit should have a small division of blackened card at 

 F, to prevent the possibility of the lights mingling with each 

 other, and thus affecting the accuracy of the result. 



In using this instrument, place it in the same straight line 

 between two antagonist flames, at the distance of six or eight 

 feet from each other ; move it nearer the one or the other, till 

 the disc of paper appear equally illuminated on each side of the 

 middle division, and the illuminating powers of the flames will 

 be directly as the squares of their distances from the middle of 

 the photometer. In moving the instrument rapidly between the 

 two lights, we very soon discover a boundary, on each side of 

 which the difference between the illuminated discs becomes quite 

 apparent. By making the instrument move from one side of 

 this line to the other, and gradually diminishing the lengths 

 of the oscillations, we at last place it almost exactly in its proper 

 position. It is very convenient to have a board of the same 

 breadth with the instrument, divided into equal parts, for the 

 purpose of supporting it, and reading off with ease and accu- 

 racy the distances of the flames from the middle of the instru- 

 ment. 



In viewing the illuminated disc of paper, I use a box, about 

 eight inches long, in the form of a prismoid, and blackened with- 

 in, in order to prevent any light entering the eye, except what 

 passes directly through the disc of paper. 



Instead of the two mirrors, I sometimes use the same instru- 

 ment, with a piece of white paper pasted on the faces of the 

 mirrors, or on a piece of smooth wood, forming, as before, a right 

 angle. In this case, the illuminated discs are viewed directly 

 through the rectangular opening in the lid, without the inter- 

 vention of the tissue or oiled paper. 



This instrument is still simpler than the preceding, and in 

 some experiments has decided advantages. But whatever form 

 of the instrument be employed, the following precautions should 



