selenium, by the simple process of rubbing
a stick of this substance over the edges, and afterward gently warming.
The selenium then presents a grayish appearance, and is ready for use.
Connection is made by conductors, on opposite sides, with the odd and even
numbers of the disks, which diminishes the resistance of the selenium. The
cylinder thus formed is insulated by glass supports in the inside of a
vacuum tube, for the purpose of preserving it from the disturbing
influence of dark rays. The whole is placed upon a stand, and shielded
from reflected light, but fully exposed to that which is to be measured
for actinic intensity. If now a constant current of electricity is passed
through the apparatus, as indicated by a galvanometer, the variations of
the latter will show the effect produced upon the selenium. A scale must
be prepared, with the zero point at the greatest possible resistance of
the selenium, which corresponds with absolute darkness. The greatest
effect of the light would be to annul the resistance of the selenium.
Consequently, the cylinder must be withdrawn from the circuit to represent
this effect; and the maximum deviation of the galvanometer is then to be
observed, and marked 100. By dividing the range of the galvanometer thus
obtained into 100 equal parts, the requisite actinometric scale will be
established. In practice, the Clamond battery is used to supply the
constant current required.

* * * * *

ASTRONOMICAL PHOTOGRAPHY.

During the last few years, or rather decades of years, it has become
rather a trite saying that to advance far in any branch of physical
research a fair proficiency in no inconsiderable number of the sister
sciences is an absolute necessity. But if this is true in general, none, I
think, will question the assertion that a proficient in any of the
physical sciences must be fairly conversant with photography as a science,
or at least as an art. If we take for example a science which has of late
years made rapid strides both in Europe and America, the science of
astronomy, we shall not have far to go to find convincing proof that a
great portion of the best work that is being done by its votaries is
effected by the aid of photography. One eminent astronomer has quite
lately gone so far as to declare that we no longer require observers of
the heavens, but that their place can be better supplied by the gelatine
plate of the photographer;