ILLUMINATION 



885 



them about equal In breadth to either shadow. The brighter flame must then be 

 removed, or the feebler brought nearer to the screen, till the two shadows appear of 

 equal intensity, when their distances from the lights must be measured, and their 

 total illuminating powers will be in the direct ratio of the squares of the distances. 



Ritchie's Photometer consists of a rectangular box, about an inch and a half or two 

 inches square, open at two ends. It is blackened within to absorb the extraneous 

 light. Within, inclined at angles of 45 to its axis, are placed two rectangular pieces 

 of plain looking-glass, cut from one and the same rectangular strip ; these are fastened 

 so as to meet in the middle of a narrow slit, about an inch long, and an eighth of an 

 inch broad, which is covered with a slip of fine tissue or oiled paper, and a blackened 

 card prevents the reflected images from mingling. If we would compare two lights, 

 they must be placed at such a distance from each other, and from the instrument be- 

 tween them, that the light from every part of each shall fall on the reflector next it, 

 and be reflected to the corresponding portion of the paper. The instrument is then to 

 be moved nearer to the one or the other, till the paper on either side of the di visioa 

 appears equally illuminated. When the lights are thus exactly equalised, it is clear 

 that the total illuminating powers of the luminaries are directly as the squares of 

 their distances from the middle of the instrument. 



Wheatstone's Photometer is a small sphere with a reflecting surface, which being 

 placed between the two lights, each light is seen on it by the spectator, the two being 

 reflected from different points of the sphere's surface. By an ingenious but simple 

 mechanical contrivance, a rapid looped motion is communicated to the ball, and by 

 the principle of the persistence of impressions, the spectatoi immediately sees two 

 looped curves of different brightnesses. The brighter light is removed until these 

 curves seem of the same brightness, and the intensities of the luminous points are then 

 as the squares of the distances. 



Bunsen's Photometer consists of a sheet of cream-coloured letter-paper, rendered 

 transparent over a portion of the surface by a mixture of spermaceti and rectified 

 naphtha, which is solid at common temperatures, but becomes liquid on the application 

 of a very gentle heat. The mixture is liquefied and painted over the paper with a. 

 brush, leaving a round disc of the size of half-a-crown in the centre uncovered. "When 

 a light is placed on one side of the paper a dark spot is observed on the uncovered 

 portion. When another light is placed on the other side of the paper, the spot is still 

 distinctly visible, if the distance of the light is such that the reflected portion from the 

 paper be either of greater or of less intensity than that transmitted. When the paper 

 is so situated between the two flames that the transmitted and reflected light are of 

 the same intensity, the uncovered spot is no longer visible. 



It will be evident from these descriptions that it is possible only, by any of these 

 contrivances, to compare one light with another ; there is not any arrangement by 

 which we are enabled to express absolutely the illuminating power. Upon the 

 principle of comparison, and comparison only, the following Tables have been con- 

 structed by the relative experimentalists. The observations of Pe*clet have been 

 already given. The following comparative view of wax and stearine candles manu- 

 factured in Berlin, which have been deduced from the observations of Schubarth, is of 

 much value: 



