48 PRACTICAL PHOTO-MICROGRAPHY. 
is a matter of choice, being regulated by the brake c; 
but if great tension is put on the shutter there is danger 
of jar and blurring, though we find no such trouble 
down to the tenth of a second. 
Having at one time to photograph a certain object 
changing its form several times in a few minutes, but 
not requiring very rapid exposures, we adapted to this 
camera a long “ repeating back” I) (fig. 13), carrying 
four lantern-slide-sized plates side by side, and were 
able by pushing this slide along as required to get 
four negatives at suitable intervals, about thirty seconds. 
Having had this instrument constructed by Messrs. 
Watson and Son, of High Holborn, we discovered that 
it did away with the necessity for a heliostat when 
using direct sunlight ; for while observing the object on 
the ground-glass of the brass tube we could manipulate 
all the illuminating apparatus, including a second 
mirror which was placed so as to illuminate the 
substage mirror of the microscope. It need not be said 
that the image of the sun remained sufficiently steady 
for the short exposures required for such a radiant. 
Fig. 14 shows diagrammatically the nature of the 
shutter device inside the camera. 
Even with limelight we have made good nega¬ 
tives of 350 diameters magnification, the exposure being 
about one-fifth of a second. 
Apparatus for Large Negatives, and Very Low 
Magnification. 
To the ordinary worker in photo-micrography, who 
has to face all kinds of work that may present them- 
