Dr. W. E. Sumpner on the Diffusion of Light. 85 



Taking as an average case for rooms a reflecting power of 

 70 per cent, for the ceiling, 40 per cent, for the walls, and 

 20 per cent, for the floor, the average value of rj is a little 

 over 40 per cent., and the increase of illumination by re- 

 flexion becomes as much as 70 per cent. If the walls and 

 ceiling of a room be well whitewashed, the average reflecting 

 power will not fall far short of 80 per cent.; and in such 

 cases the illumination due to diffused reflexion is four times 

 as important as that caused by the direct action of the lights 

 in the room. A further great advantage, resulting from the 

 use of good diffusing surfaces, arises from the fact that the 

 illumination they cause is, in most cases, very approximately 

 constant all over the room, and does not cast shadows. 



It is to be noted that when a reflecting surface is coloured, 

 its average reflecting power does not properly represent the 

 character of the increase of illumination caused by it. A 

 room whose walls are covered with red paper whose average 

 reflecting power is 40 per cent., may quite possibly have the 

 red light in the room increased five times owing to the action 

 of the walls. Suppose, for instance, that the reflective power 

 of the paper for red light is 80 per cent., and for other kinds 

 of light only 10 per cent., the average reflecting power will 

 not exceed 40 per cent., yet the red light will be increased 

 five times, while other kinds of light will not be increased to 

 any perceptible extent. 



Measurements of Reflective Power, 



The surface, whose reflective power was required, was 

 attached to a large screen of black velvet placed at one end 

 O of a 3-metre photometer-bench OL (see fig. 2), so as to 



Fig. 2, 



be perpendicular to its length. Two lights were used, one 

 being a Methven two-candle gas standard, placed at L, and 



