486 WRITINGS OF JOSEPH HENRY. [1875 



the two flames may be immediately read off in terms of can- 

 dle-power, when a small intermediate movable screen is 

 equally illuminated on both sides. This screen is usually 

 formed of a piece of white pasteboard of about four inches 

 square, fixed perpendicularly at right angles to the length of 

 the scale in a sliding frame, by which it can be brought 

 nearer to or farther from one of the lights. In the centre of 

 this square is a circular hole of about half an inch in 

 diameter which is closed by a piece of thin paper, rendered 

 translucent by a solution of spermaceti in oil of turpentine. 

 This forms a spot which is darker than the other parts of the 

 white screen, and is equally dark on both surfaces when the 

 screen is receiving an equal quantity of light from each 

 flame; the screen is moved backward and forward until 

 this efiect is produced, and the index will then point on the 

 graduated scale to the number of the relative power of one 

 of the lights in the terms of the other. 



The screen may also be made of thin paper, the whole of 

 which is rendered translucent except a round spot in the 

 centre of half an inch in diameter. If a light is placed before 

 the screen on one side, the whole of the greased part will 

 appear dark, on account of part of the light going through 

 the translucent portion. If now another light be placed on 

 the opposite side an equal portion will be transmitted 

 through the pellucid part, and the two surfaces will appear 

 of like intensity when the two lights are equal, or when from 

 their respective distances they throw equal amounts of light 

 on the two faces of the screen. 



In order that both sides may be seen at the same moment 

 without moving the head from one edge of the screen, two 

 mirrors making with each other an angle of 90° are placed 

 so that the screen itself will bisect the angle. 



For dividing the scale into parts related to each other as 

 the square of their distances from a centre, the following 

 formula and table will furnish the means. Let a be the 

 length of the scale, and x the distance from the caudle end 

 to the movable screen; then a — x is the distance between 

 the lamp end and the screen. Denote the degree of illu- 

 mination on the candle and lamp sides of the screen 



