VOL. LXXXIV.] PHILOSOPHICAL TRANSACTIONS. 36l 



That the shadows being of equal density at any given point, the intensities of 

 the illuminating rays inust of necessity be equal at that point also, is evident from 

 hence, that the total absence of light being perfect blackness, and the shadow 

 corresponding to one of the lights in question being deeper or fainter, according 

 as it is more or less enlightened by the other, when the shadows are equal, the 

 intensities of the illuminating rays must also be equal. When the intensity of 

 one strong light is compared with the intensities of several smaller lights taken 

 together, the smaller lights should be placed in a line perpendicular to a line 

 drawn to the centre of the paper, and as near to each other as possible; and it is 

 also necessary to place them at a greater distance from the paper than when only 

 single lights are compared. In all cases it is absolutely necessary to take the 

 greatest care that the lights compared be properly trimmed, and that they burn 

 clear, and equally, otherwise the results of the experiments will be extremely 

 irregular and inconclusive. 



To ascertain by this method the comparative densities, or intensities of the 

 light of the moon, and of that of a candle, the moon's direct rays must be 

 received on a plane white surface, at an angle of incidence of about 6o°, and the 

 candle placed in the line of the reflection of the moon's rays from this surface; 

 when the shadows of the cylinder corresponding to the moon's light, and to that 

 of the candle, being brought to be of equal density, by removing the candle 

 farther off, or bringing it nearer to the centre of the white plane, as the occasion 

 may require, the intensity of the moon's light will be equal to that of the candle 

 at the given distance of the candle from the plane. To ascertain the intensity of 

 the light of the heavens by day or by night, this light must be let into a dark- 

 ened room through a long tube, blackened on the inside, when its intensity may 

 be compared with that of a candle or lamp by the method above described. To 

 determine the intensity of the direct rays of the sun, compared to the light 

 emitted by any of our artificial illuminators, it may perhaps be necessary, consi- 

 dering the almost inconceivable intensity of the sun's light, to make use of some 

 further contrivances and precautions. And when the relative intensity of the 

 sun's light at the surface of the earth, compared with the intensity of the light 

 of a given lamp, placed at a given distance, and burning with a flame of given 

 dimensions, shall be known; it will then be easy, from the known size and dis- 

 tance of the sun, to compute the relative density of his light at his surface, com- 

 pared to the density of the light of the flame of the lamp at the surface of that 

 flame. The intensity of the light emitted in the combustion of iron or of phos- 

 phorus in dephlogisticated air, as also that of all other burning, or red-hot bodies, 

 may be compared and determined by this method with the greatest facility and 

 exactness. 



In pursuing the experiments. Count R. found it convenient to make several 



VOL. XVII, 3 A 



