of the Light emitted by Luminous Bodies. 17 



100 inches from the centre of the field of the photo- 

 meter, measuring from the centre of the circular flame 

 of the lamp, a cylindric wax candle, of known weight 

 and dimensions, and which is kept merely for that 

 purpose, being lighted and trimmed, and made to burn 

 with the greatest possible degree of brilliancy, is placed 

 over against it, at a certain given distance (33 inches), 

 and then the wick of the lamp is drawn out or short- 

 ened, as it is found necessary, till the shadows corre- 

 sponding to the lamp and to the candle are precisely 

 of the same density : this done, the proof candle is ex- 

 tinguished, and laid by for further use, and the pro- 

 jected experiment is immediately commenced. 



Here the proof candle is, properly speaking, the 

 standard; but the lamp is to be preferred to it, for the 

 experiments, on account of the superior constancy or 

 equality of its light. 



The only danger of error in this mode of proceeding 

 arises from the difficulty of procuring proof candles 

 which shall always give precisely the same quantity of 

 light, or of making the same candle burn with exactly 

 the same brilliancy at different times. I flattered my- 

 self at one time that even this cause of error and 

 uncertainty, however insurmountable the difficulty ap- 

 pears, might be in a great measure removed. I con- 

 ceived that if the light from the standard lamp and 

 that of the proof candle, brought to be of the same 

 intensity at the surface of the vertical plane, were 

 really stronger at one time than at another, the equal 

 shadows of the cylinders would be proportionally 

 deeper, and that by comparing at different times the 

 density of those shadows with a painted scale of 

 shades, regularly graduated, any difference in the in- 



