68 Royal Society :— 
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 effected 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 effected by both direct and diffuse solar light; and 
Column III. the same action expressed in light-metres :— 
1g IL. Ill. 
Melville Island ...... 1196 11790 1306 metres. 
Reykiavik .......... 5964 20980 D304 ae 
St. Petersburg ...... 8927 25340 D306: | Tap 
Manchester.......... 14520 32740 3020 sae 
Heidelberg. ........ 18240 37340 AV30) 1 iiss 
Naples. 5. ...+00500. 20040 47190 5o20gn as 
WANG, Vee eos wisi ed ee OOLSY 58110 6437 Cs, 
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 imcan- 
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, 7.e. the effect produced on the eye, of 
the two sources in question. ‘Thus, when the sun’s zenith distance 
was 67° 22', the chemical brightness of that source was 36:6 times, 
but the visible brightness 525 times as large as that of the terrestrial 
source of light. 
In the last section of this communication the chemical action of 
the constituent parts of the solar spectrum is investigated. The 
sun’s rays were reflected from a Silbermann’s heliostat, and after 
passing through a narrow slit, they were decomposed by two quartz 
prisms. The spectrum thus produced was allowed to fall upon a 
white screen covered with a solution of quinine, and any desired por- 
tion of the rays could be measured by a finely-divided scale, and the 
position noted by observation of the distances from the fixed lines. 
For the purpose of identifying the fixed lines in the lavender rays, 
the authors were, by the kindness of Mr. Stokes, allowed the use of 
an unpublished map of the most refrangible portion of the spectrum, 
prepared by that gentleman. As the various components of white 
a ta alia i i 
