94 
Passing from Kew to Para, it appears that the chemical action of 
total daylight during the month of April, 1866, at Para was 6.6 times 
as great as at Kew. 
In order to obtain data for a clearer atmosphere, Roscoe and 
Thorpe conducted observations in 1867 near Lisbon, Portugal, and 
published their results in a memoir of 1870, where they have given 
the relation between the sun’s altitude and the chemical intensity. 
The intensity is the same for hours that are equidistant from ap- 
parent noon. The relative intensity of direct sunlight, reflected sky 
light, and total insolation is shown for different altitudes at Lisbon 
by the following table: 
Intensity of insolation at Lisbon for clear skies. 
Menor ar een Observed a a inten- | 
titude of jof obser- 
Soe acne. | Sansa ein toil: 
: | 
9.85 15 | 0.000 | 0.038 | 0.088 
19.68 18 023 062 O85 
31.28 22 | 052 100 | 152 
42.22 22 |  .100 Tab Als Pets 
53.15 19 136 126 | 262 
61.13 2% | 195 182 | 827 
64.23. | u 221 138 | .359 
| i} 
In general, the total intensity is directly proportional to the num- 
ber of degrees of altitude. For altitudes between 18° and 35° the 
intensity on a plane perpendicular to the incident rays is about the 
same as the intensity of total sky hight on a horizontal plane. The 
intensity of direct sunlight on a horizontal plane is equal to the 
intensity of total sky light on a horizontal plane when the sun’s alti- 
tude is about 45°. At all altitudes of the sun below 21° the chemical 
action of diffuse daylight exceeds that of direct sunlight. 
In their memoir of 1871 Roscoe and Thorpe determined the amount 
of chemical action for total sky light of a cloudy sky during totality 
of the solar eclipse, and found it much less than 0.003, and therefore 
not measurable. They found the total chemical action of the direct 
sunlight to be strictly proportional to the visible area of the portion 
of the solar disk up to a certain point in the obscuration, after which 
the influence of sky hght is inappreciable. For altitudes below 50° 
at Catania, Sicily, as elsewhere, the amount of chemical action 
effected by diffuse dayhght on a horizontal surface is greater than 
that exerted by the direct sunlight. At altitudes less than 10° direct 
sunlight is almost completely robbed of its chemically active rays. 
