on the Northern Hemisphere at Summer Solstice. 359 



North Pole, and at 30° X. lat. the total quantity of chemical 

 light has not quite twice the value it has at the pole. Add 

 to this, that at 73°*5 X. lat. (that is, more northerly than 

 the north of Norway) the total quantity of chemical light is 

 as great as at the equator, and further, that for a strip from 

 55° t<> 73°'5 X. lat. the daily quantity of light on the said 

 day i< as great as for an equatorial range to ahout 7° N. lat. 

 When we go towards the north from the 55th degree ot 

 latitude the total quantity of chemical light will increase by 

 some thousands of light-units until a secondary maximum at 

 60° X. lat. is reached. A very great decrease of about 

 10,000 chemical light-units, or about one-eighth of the 

 total quantity of light at the equator, having taken place 

 from 50 c to 55° X. lat., there appears once more an increase 

 of the quantity of light for a range reaching from the south 

 coasts of the larger Danish isles to the Norwegian town 

 Kristianssund. From here the quantity of light decreases; 

 further, but does not within the Scandinavian peninsula 

 attain as low a value as at the equator. Even for the 

 northern part of Scandinavia (70° X. lat.) the total quantity 

 of chemical light on midsummer-day is not less than 80 per 

 cent, of its maximum value at 30° X. lat. 



We have thus shown that the northern parts of the globe, 

 with regard to the chemical light that they receive at mid- 

 summer-time under a perfectly pure and cloudless atmosphere, 

 are much more fortunately situated than we should have 

 expected from the relative numbers in the table of Spitaler. 

 According to these, we should find that the quantity of light 



2 SO 

 on a day of June at 70° X. lat. would be ^ . 100 = 58 per 



cent, of the quantity of light on the same day at 30° X. lat., 

 where we have the maximum even according to Spitaler. 

 In other respects, too, there are essential differences between 

 the numbers calculated from the formulae of Bunsen and 

 Roscoe and those of Spitaler. Thus we do not find in the 

 numbers for the actinic light that increase of the quantity of 

 light that according to Spitaler should take place from 70 c 

 to90 c X. lat. 



-V- we have already mentioned, we have only referred 

 to that part of the sunlight that produces the well-known 

 effect on the explosive mixture of hydrogen and chlorine. 

 Tii'- -aid effect has its maximum value in the spectral range 

 |GH — H, and is produced chiefly by the violet and ultra- 

 violet ray-. 



The effect of the direct sunrays of other wave-lengths i> 



