photosynthetic activity. When there is no light, the plant organisms which are the original source 

 of food for all the rest of the organisms, cannot develop. 



Secondly, under-ice illumination is connected with the practical activity of man: diving work 

 and navigation under ice pertain to this matter. 



Unfortunately, under-ice illumination has been poorly studied and the available observations 

 are fragmentary, sometimes even inconsistent. 



Trofimov, when studying the conditions of under-ice illumination in the White Sea in April 

 1934, computed the conditional ice transparencies as the ratio of the intensity of illumination 

 under the ice to the intensity of the light incident from above, minus the albedo of the surface. 



The results of his observations are shown in table 55. 



TABLE 55. ILLUMINATION UNDER ICE AND SNOW OF DIFFERENT THICKNESSES (IN CM) 



The "Non-albedo" transparency of ice in this table was computed according to formula 



where I is the illumination imder the ice, 

 A is the albedo, 

 i is the thickness of the ice in meters. 



100— A ' 



As can be seen from the table, the transparency of the ice computed for 1 m of thickness 

 fluctuated within the limits of 0. 6 to 37 per cent which is explained by the usual non-homogeneity 

 of the ice cover under natural conditions. 



Trofimov' s special observations of individual plates of ice 1 to 2 square m in area which had 

 lain for a sufficient time in the air (ropaki) and which are almost fresh (salinity less than 1 o/oo), 

 gave the results shown in table 56. 



The icefloes which were investigated were lilac blue which indicated the high purity of the ice. 



178 



