170 TRANSACTIONS OF THE 
the value of k, and the outside intensity adjacent to the 
room, it is possible by certain empirical ‘formule to 
arrive at a close approximation of the number of lamps 
of a given capacity to compensate for any deficiency in 
daylight. 
For example, in the problem above, we found that 
the intensity on the last row of desks was about 4.4 ft. 
candles. Suppose it were required to increase this to 7.5 
ft. candles, thus making 3.1 ft. candles to be provided for. 
By the empirical equation N=AI/LE, where A= area of 
the room, I=illumination per sq. ft. L—illumination per 
lamp and E= utilization factor, i. e. the effective illumin- 
ation to be considered, then N, the number of lamps, is 
readily calculated, thus avoiding the wasteful tendency 
of guessing at the number of lamps to install. Usually 
more lamps are installed than are necessary in order to 
be sure that there shall be sufficient light for the worst 
possible conditions. This is not only uneconomical, but it 
is positively injurious since! too much light produces 
brilliancy and leads to a dazzling effect, thus injuring 
the eyes. But the problems of brilliancy and dazzling 
lie outside the range of this investigation and cannot be 
considered here. 
Perhaps it has already been noticed that no atten- 
tion has been paid to absolute values either in foot candle 
illumination or per cent of daylight factor. This study 
was primarily made to work out the relations which have 
been given above, that being deemed the problem of first 
importance. Absolute values may come later. 
