464 Dr. C. H. Lees on the Intensity of Illumination 



small area, as, for example, an Argand gas-flame or an arc- 

 light, symmetrical about an axis perpendicular to the plane, 

 will, I trust, be found simpler to understand and easier to 

 carry out in practice than that proposed by Loppe, and quoted 

 by Palaz, in his Photometrie Industrielle, p. 253. 



The source of light is supposed to be at a distance from 

 walls &c. producing diffuse reflexion, but it may be provided 

 itself with reflectors, if the photometer-curves for the source 

 are taken with the reflectors in position. The most important 

 cases in practice are those in which the illuminated plane is 

 horizontal — in general the ground ; and the source of light is 

 symmetrical about a vertical axis. 



Let A, B, C, D be the photometer-curve of the given source 

 of light at L, the distances L A and L B &c. being equal, on 

 some convenient scale indicated by the figures 15 or 700 on 

 the circles, to the intensity of light sent out by the source in 

 the directions L A, L B, &c. 



Fig. 1. 



Let the illumination of the horizontal plane G E through E, 

 produced by the source L, be required. » 



Draw a line M L through the source L parallel to tbe 

 plane GE. 



Through any point D, say, of the photometer- curve, draw 

 D F parallel to the horizontal plane GE. 



Join D L and produce to meet the horizontal plane in G. 



Through F draw F H perpendicular to D L, and through H 

 draw H K parallel to G E, meeting the vertical line G M in K. 



The illumination at G is then equal to MK/(MG) 2 , MK 

 being measured on the same scale as the radii of the photo- 

 meter-curve. 



