176 W. K. BRIDGMAN ON ILLUMINATION, &C. 



quantity, as light in its proper state can hardly be too abundant ; for 

 it is only when contaminated with the unnecessary rays that it be- 

 comes distressing to the eyes. Then, as regards intensity, it must 

 be borne in mind that light, like its correlatives, varies inversely as 

 the square of the distance ; so that at two inches distance it becomes 

 four times less than at one inch, and at four inches, sixteen times, 

 and so on in like proportion. Hence, to increase or diminish intensity, 

 we have only to adjust the distance of the lamp accordingly, and we 

 may by these means dispense with ground or tinted glasses, both of 

 which materially interfere with delicate vision. 



