270 



PHOTOGRAPHY 



and close the blinds in order to keep the house cool, because the 

 exclusion of light means the exclusion of some heat ; in winter 

 we open the blinds and raise the shades in order that the sun 

 may stream into the room and flood it with light and warmth. 

 Our illumination, whether it be oil lamp or gas jet or electric 



light, carries with it heat; 

 indeed, so much heat that 

 we refrain from making a 

 light on a warm summer's 

 night because of the heat 

 which it unavoidably fur- 

 nishes. The heat of the sun 

 and the light of the sun 

 seem boon companions. 

 The heat of the sun as well 

 as the light of the sun is 



FIG. 138. Vegetation in northern Russia. . . . i r 



The trees are nearly 100 years old. essential to the growth Ol 



plants. If spring, with its 



warm days, is late, seeds are late in sprouting ; if the summer 

 is cold, crops are slow and poor. Indian corn will not mature 

 and give full big ears unless hot sun shines down on the field 

 in its growing season. Far north, where the sun's rays bring 

 little heat to the land, plants are few and stunted (Fig. 138). 

 In the polar regions, where the temperature is always low, 

 plants do not grow at all. 





