94 



Passing from Kew to Para, it appears that the chemical action of 

 total dajdight 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. 



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 light 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 0.008, 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 light is inappreciable. P"or altitudes below 50° 

 at Catania, Sicily, as elsewhere, the amount of chemical action 

 effected by diffuse daylight 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. 



