199 MARVELS OF FISH LIFE 
Working with an are lamp of 1,200 candle-power, 
and with a cooling tank between the light and the 
object to be photographed, the usual exposure up to 
twenty-five magnifications on a slow plate is one-fifth 
of a second. Magnification above this requires one to 
three seconds. The photograph of oyster spat on the 
plate facing p. 148 shows a micro-photograph of the 
living moving oyster spat magnified sixty times, and 
taken at one-tenth of a second. When it is desired to 
take a photograph from life size up to ten magnifica- 
tions, the microscope is slipped out, and a tank is fixed 
in place of the former, as illustrated. All the photo- 
graphs, such as the embryo thornback ray, the hatching 
salmon, and the developing roach, were taken in this 
manner. 
The reason for the great length of bellows is that it 
is possible to get considerable magnifications with lenses 
of comparatively long focal lengths ; in this way objects 
that have considerable thickness are sharp in focus all over. 
It is not within the province of a book of this character 
to deal at length with the various ways of manipulating 
artificial light, or to describe the various modifications 
of live cells that I use, but I would briefly refer to a 
method I employ of obtaining a true rendering of trans- 
parency in marine larve and crustaceans. Take, for 
example, a small crustacean. If the light is merely 
transmitted through this animal into the lens, the 
result obtained in no way suggests the transparency 
of the creature. (See top photograph in the opposite 
