MR RITCHIE on a new Photometer. 443 



f ' <t -S. f 







XXXII. On a new Photometer, founded on the Principles of 

 Bouguer. By WILLIAM RITCHIE, A. M. Rector of Tain 

 Academy. Communicated by Dr BREWSTER. 



' 



(Read May}. 1826.) 

 



AHE celebrated BOUGUER was the first who discovered the 

 important fact, that the eye can detect a very small difference 

 between two similar illuminated surfaces, when viewed at the 

 same moment, the only principle which has yet been applied 

 with any degree of success, in determining the relative illumi- 

 nating powers of artificial flames. The following is perhaps the 

 most commodious application of this principle, to determine the 

 relative illuminating powers of different artificial lights, particu- 

 larly of coal and oil gas. The instrument, or photometer, which 

 I employ for this purpose, is extremely simple. It consists of a 

 rectangular box, about an inch and a half, or two inches square, 

 open at both ends, and blackened within for the purpose of 

 absorbing the stray-light. Within the box are placed two rec- 

 tangular pieces of plane mirror, forming a 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, about an inch long, and one-eighth of an 

 inch broad. This opening is covered with a slip of fine tissue 

 or oiled paper. In the annexed figure, 

 A B C D is the box ; C F, F D, the two 

 plane mirrors ; E G the rectangular open- 

 ing covered with a small disc of oiled or 



fine paper. I need hardly mention, that c D 



the two mirrors should be cut from the same plate, in order 

 that their reflective powers may be exactly equal. The rec- 



