460 Messrs. Richardson and Quick on a Form of 



is placed specially prepared sensitized paper. Before the 

 pendulum is set vibrating, the shutter is in such a position 

 that it completely covers the slit, one complete vibration 

 causing the shutter to be withdrawn and to be again returned 

 to its original position. The edge of the slit is graduated into 

 millhns., and the time during which each division is opened 

 is calculated. The different parts of the paper are thus 

 exposed for known and varying times, a graduated tint of 

 darkening being produced. The shaded paper is then com- 

 pared with the standard tint, and the point where the two 

 shades are equal is determined. From this the time of ex- 

 posure can be found, since the degree of darkening is directly 

 proportional to the time of exposure within wide limits. 

 The unit of actinic intensity taken is that intensity of light 

 which will produce, on such sensitized paper, a degree of 

 darkening equal to a standard tint in one second of time. 

 The reciprocal of the above time of exposure thus gives the 

 intensity of light at the time of the experiment. 



The present modification was devised in order that the 

 process of opening and shutting the slit might be brought 

 about by a motion applied to the shutter in one direction only. 



Fiff. 1. 





A 



c 



.1 





H 



B 



0v 







Id) 



Ai) P- 



K 





— K J 



L 



Fig. 1 shows the arrangement by which this 

 is attained. 



The shutter AB consists of a wide strip of 

 thin, black, waterproof, fabric in the form of 

 an endless band, running over two wooden 

 rollers C D ; its upper and lower surfaces being 

 brought just into contact by means of two 

 additional smaller rollers EF, so that the two 

 layers move freely past each other in opposite 

 directions when set in motion. 



An aperture GH, 20 centim. long and 1 centim. wide, is cut 

 in each surface, in such a position that the two openings 

 exactly coincide when midway between the rollers. The 

 sensitized paper is contained in the slide I, the top of which 

 is made of very thin sheet ebonite, over which the shutter 

 runs, the edges LL of the ebonite being bent down to prevent 

 any possibility of the edges of the shutter catching as they come 

 round. A slit J (fig. 2), 10 centim. long and 1 centim. wide, 

 is cut in the ebonite, beneath which the paper is exposed ; 

 the latter is fastened to the board K K, by means of which 





