108 TRANSACTIONS OF THE | JAN. 16, 
line, might have been slower or faster, perhaps, without affect- 
ing the result. ‘The record obtained in each case is a line showing 
nearly three waves, and closely corresponding in this respect to 
the front shutter records, but tapering to infinity at either end, 
precisely like the lines obtained by Londe. 
One of these records is shown in Fig. 9; and beyond the end of 
Fie. 9. 
ORS TORU Nae Ny 
the primary line is seen a supplementary figure, which indicates 
« double exposure caused by the rebound of the front shutter, 
and an almost simultaneous rebound of the back shutter. With 
rapid shutters, the absence of this indication without the spring 
catch isexceptional. By thesymmetry or distortion of this figure, 
the relative movements of the two shutters may be inferred. 
From these records it appears that the inner shutter contains 
very nearly the same number of undulations and indicates very 
nearly the same time of exposure as that upon the outer shutter. 
They are made separately, and as absolute constancy in the rubber 
spring can hardly be expected, a slight difference between them 
might occur. But tapering as it does to infinity at each end, the 
line in the last record indicates that the effective exposure with 
the lens employed is somewhere about one-eighth or more less 
than the shutter exposure, for the light which so faintly depicts 
that proportion of its terminals would constitute but an indif- 
ferent proportion of the total actinic effect on the plate. In this 
case, a surplus of light was used, and Londe has shown that this 
line, descending as it does in intensity from an intermediate maxi- 
mum, would reach infinity, and terminate, with a weak light, 
further from both the beginning and end of the exposure. This 
relation between the lens and the intensity of the light may be 
shown, perhaps, quite as well without a vibrating beam. In 
Fig. 10 is an actual record of an immovable point of light upon 
Fia. 10. 
the inner shutter, the black spots indicating the limit of ex- 
cursion of the shutter having been first made by special ex- 
posures. To draw comparison between two different records 
thus obtained, however, the velocity of the,back shutter must 
remain unchanged. 
