41 



of error are eliminated, it is possible, by means of this standard 

 flame, to reduce the indications of different instruments to the same 

 unit of luminous intensity, and thus to render them comparable. 

 For this purpose, the authors define the photometric unit for the 

 chemically active rays, as the amount of action produced in one 

 minute, by a standard flame placed at a distance of one metre 

 from the normal mixture of chlorine and hydrogen; and they 

 determine experimentally for each instrument the number of such 

 units which correspond to one division on the scale of the instru- 

 ment. By multiplying the observed number of divisions by the 

 number of photometric units equal to one division, the observations 

 are reduced to a comparable standard. It is proposed to call this 

 unit a chemical unit of light, and ten thousand of them one chemical 

 degree of light. 



According to this standard of measurement, the chemical illu- 

 mination of a surface, that is, the amount of chemically active light 

 which falls perpendicularly on the plane surface, can be obtained. 

 It has thus been found that the distance to which two flames of 

 coal-gas and carbonic oxide, each fed with gas at the rate of 4*105 

 cubic cent, per second, must be removed from a plane^surface, in order 

 to effect upon it an amount of chemical action represented by one 

 degree of light, was, in the case of the coal-gas flame, 0'929 metre, 

 in that of carbonic oxide 0'561 metre. The chemical illuminating 

 power, or chemical intensity, of various sources of light, measured 

 by the chemical action effected by these sources at equal distances 

 and in equal times, can also be expressed in terms of this unit of 

 light ; and these chemical intensities may be compared with the 

 visible light-giving intensities. In like manner, the authors define 

 chemical brightness as the amount of light, measured photochemi- 

 cally, which falls perpendicularly from a luminous surface upon a 

 physical point, divided by the apparent magnitude of the surface ; 

 and this chemical brightness of circles of zenith-sky of different sizes 

 has been determined. Experiment shows that the chemical bright- 

 ness of various sized portions of zenith- sky, not exceeding 0'00009 

 of the total heavens, is the same ; or, that the chemical action 

 effected is directly proportional to the apparent magnitude of the 

 illuminating surface of zenith-sky. 



It is, however, important to express the photochemical actions 



