1888. ] NEW YORK ACADEMY OF SCIENCES. 109: 
From these indications we may advance into the realm of 
speculation, and, perhaps, discern a possible means of adjusting 
an exposure by a scale of tensions to the available light. Todo 
this, we should, perhaps, determine by Londe’s method the 
number of undulations traced by the lens upon the back shutter, 
by a front shutter exposure determined by my method with a 
full light. Then with a medium light slow up the front shutter 
until by successive exposures a similar number of undulations be 
obtained. Note the lesser tension required, and determine that 
corresponding to a light requiring the slowest exposure appli- 
cable to moving objects. The adjustment of the tension, which 
we now do by intuition, might possibly for that shutter and lens 
be thereafter done by a scale of actual velocities, only an in- 
variable spring being required, and one element of uncertainty 
in instantaneous exposures be thus avoided. Theshutters most 
generally used are, however, not simply drop shutters, but are 
so constructed as to move in an arc or circle, and in some cases 
within an enclosure which prevents the attachment of a film. 
One of these shutters, reputed to be very rapid, consisting of 
two circular apertures passing in opposite directions, and open- 
ing and closing from the centre at the diaphragm of an optimus 
lens, I have examined by the modification of Londe’s method 
last described, and the interesting records obtained are shown 
together in Figs. 11, 12, 13 and 14. Fig. 11 was obtained with 
Fias. 11, 12, 13, 14, 15. 
Nes NS \ 
SFG TNS 
|, eer TON 
LS ANIN 
a beam of normal sunlight. Fig. 12 with a beam of condensed 
sunlight, and both with a similar slow exposure. The double 
