28 ON ILLUMINATION, 



Before this white surface, at the distance of S^V Inches, are 

 placed two little pillars of wood, painted black, 4 inches inhelght 

 and half an inch in dianrieter. These little pillars, which are 

 cylindrical, are placed at the distance of 3 -V inches asunder, 

 and they are firmly fixed in two holes formed to receive them 

 in the upper surface of the cube. They are at equal distances 

 from the black vertical line wh)';h marks the middle of the 

 white surface of the photometer, that is to say, at the distance 

 of 3 inches (English measure) from that vertical line. 

 /'hod of em- This little instrument is employed in the following manner: 

 "•■inc- that Havino; placed, in a dark room, three little tables, at the dis- 

 tance of 7 or 8 feet from each other, so as to occupy the three 

 angles of an equilateral triangle, the photometer is ])laced on 

 one of these tables, and the two lamps upon the other two ; 

 taking care that the flames of the lam})s and the middle of the 

 white surface of the photometer are of the same height, or in 

 the same horizontal plane. 



The observer being seated before the photometer, with his 

 back turned towards the lamps, he presents the photometer to 

 the two lamps, in such a manner that the direct rays from their 

 fiames fall upon the white surface of that instrument at equal 

 angles of incidence, or in such a direction that the two internal 

 shadows formed by the two pillars may touch each other witlj- 

 ont being blendedtogether, at theblack vertical line in the middle 

 of that face. As the two external shadows fall without the 

 surface of the photometer, they are of course not seen. 



When the photometer is placed, the distances of the lamps 

 are verified, and brought to a perfect equality, and when that 

 is done, the lamps are made to burn with the same degree of 

 vivacity, which is easily done by elevating a little one of the 

 wicks, or lowering the other: this must be performed by aji 

 assistant, whilst the observer keeps his eyes constantly fixed 

 upon the shadows. 



The equality of the quantities of light which the lamps emit 

 is announced by the perfect equality in the densities of the two 

 shadows which are formed in tlie middle of the white face of 

 the photometer. This is evident, because each shadow being 

 enlightened by the direct rays of the opposite lamp, if one of 

 ■the lamps gives more light than the other, the shadov/ which it 

 enlightens must of course be mere enlightened, and con^^ 



sequently 



