1 40 Mr Ritchie on a new Photometer. 



right angle with each other, and cutting the sides of the box 

 at an angle of forty-five degrees. In the upper side, or lid of 

 the box, there is cut a rectangular opening E G, about an 

 inch long, and one-eighth of an inch broad. This opening is 

 covered with a slip of fine tissue or oiled paper. The two 

 mirrors should be cut from the same plate, in order that their 

 reflective powers should be exactly equal ; and the rectangular 

 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 the distances from the mid- 

 dle of the photometer. In moving the instrument rapidly be- 

 tween the two lights, we very soon discover a boundary, on 

 each side of which the difference between the illuminating disc 

 becomes quite apparent. Bv making the instrument move 

 from one side of this line to the other, and gradually diminish- 

 ing 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 the photometer, and 

 reading off the distances of the flames from the middle of the 

 instrument. 



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

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

 within, 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 in- 

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

 the mirror, 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 intervention of the tissue or oiled paper. 



This instrument is still simpler than the preceding, and in 

 some experiments has decided advantages. But whatever 



