48 



period at which both papers become equally blackened, gives the 

 time of the phase of equal chemical intensity. Experiment proved 

 not only that these points of equality which the theory requires 

 actually occur, but also that the agreement between the calculated 

 and observed times of occurrence of the phases is very close, giving 

 proof that the data upon which the theory is founded are substan- 

 tially correct. 



The formula, by help of which the chemical action of the direct 

 sunlight eifected at any place during any given time can be calcu- 

 lated, is next developed, and the direct solar action at the following 

 places calculated for the vernal equinox from sunrise to sunset. 

 Column I. gives the action of the direct sunlight during the whole 

 day, expressed in degrees of light ; Column II. the action for the 

 same time eifected by both direct and diffuse solar light ; and 

 Column III. the same action expressed in light-metres : 



I. II. III. 



Melville Island 1 196 1 1790 1306 metres. 



Reykiavik 5964 20980 2324 



St. Petersburg 8927 25340 2806 



Manchester 14520 32740 3625 



Heidelberg ... 18240 37340 4136 



Naples 26640 47190 5226 



Cairo 36440 581 10 6437 



The authors next proceed to examine the chemical brightness of 

 the sun compared with a terrestrial source of light. For this pur- 

 pose the intensely bright light produced by a wire of magnesium 

 burning in the air was employed. Experiment showed that the 

 chemical intensity of the sunlight, undiminished by atmospheric 

 extinction, is 128 times greater than that from a surface of incan- 

 descent magnesium of like apparent magnitude ; or that burning 

 magnesium effects the same chemical illumination as the sun when 

 9 53' above the horizon, supposing of course that both luminous 

 sources present to the illuminated surface the same apparent magni- 

 tude. A totally different relation was found to exist between the 

 visible illuminating power, i. e. the effect produced on the eye, of 

 the two sources in question. Thus, when the sun's zenith distance 

 was 67 22 f , the chemical brightness of that source was 36*6 times, 



