358 Prof. J. Sebelien on the Distribution of Actinic Sunlight 



The middle curve, that indicates the distribution of the 

 actinic light from direct radiation over various latitudes, 

 shows quite another appearance. Starting from 0° N. lat., 

 the curve rises quickly and reaches its maximum value at 

 30° N. lat. From here it decreases fast, but irregularly, 

 towards the pole, changing its concavity several times. We 

 see, for instance, that at 40° N. lat. there is a small upward 

 concavity, at 50° a somewhat larger upward convexity. 

 Between 50° and 60° we find again a very strong upward 

 concavity, the curve completely ceasing to fall, and con- 

 tinuing about horizontally from 56° to 60°, that is, on a strip 

 bordered on the south by a line traversing the northern part 

 of Vancouver Island, the north of Newfoundland, Newcastle, 

 the south of Fiinen, and Zeeland, and on the north by a line 

 through the southern point of Greenland, Christiania, Upsala, 

 and St. Petersburg. From here the curve decreases again 

 towards the higher latitudes, at last showing a marked 

 upward concavity. At the said range from 56° to 60° the 

 direct insolation on midsummer-day is on the whole as large 

 as at the equator on the same day. 



While the quantity of diffused chemical light at the 

 North Pole on the said day is about double the quantity at 

 the equator, the quantity of directly insolated chemical light 

 has at the equator about double the value that it has at the 

 North Pole, and at 30° N. lat. (for instance, the Canary 

 Islands, Suez, New Orleans, Shanghai) its value is about 

 three times as great as at the North Pole. 



The value of the directly insolated chemical light (at 30° 

 N. lat.) is on the said day more than twice the maximum 

 value of the diffused chemical light (at 90° N. lat.), but at 

 the North Pole the quantity of the diffused daylight (not 

 chemical activity) will surpass the directly insolated light by 

 about 4000 chemical light-units. At 82° N. lat. the quanti- 

 ties of diffused daylight and of direct solar light are equal, 

 viz., 31,000 light-units. 



The upper full- drawn curve of the figure shows the 

 distribution of the total quantity of chemical light (W + W 2 ). 

 The aspect of this curve is obviously stamped by the curve 

 for the direct insolation. However, the diffused daylight — 

 that especially towards the north — is also greatly asserting 

 itself. Although the total quantity of light according to this 

 curve has still its maximum value at 30° N. lat., and the 

 North Pole still receives less light than the equator, yet the 

 difference between the extremes is levelled considerably. 

 A spot at the equator receives at mid summer- day only a 

 total quantity of 6000 chemical light-units more than the 



