34 PHOTO-MICROGRAPHS. 
actinic or chemical rays, which are at the violet 
end of the spectrum. This may be accomplished 
by means of a glass prism, which separates the 
rays by virtue of their different refrangibility; but 
in practice it is more conveniently accomplished 
by allowing the light to pass through a medium 
which is transparent for the actinic rays, but 
which arrests the heat rays. 
The medium commonly employed is a solution 
of the ammonio-sulphate of copper. This is a 
liquid having a deep-blue color, and is easily made 
by dissolving sulphate of copper to saturation in 
strong water of ammonia. The solution is to be 
filtered, and is placed in a flat glass cell, having 
parallel sides, which is introduced between the 
mirror and the achromatic condenser. The sides 
of the glass cell should be separated by about half 
an inch, so that the light may pass through a 
stratum of the ammonio-sulphate solution of this 
thickness. The other dimensions of the cell are 
not of material importance, but two inches square 
will be found a convenient size. 
Experience proves that it is well to introduce a 
lens of long focal distance between the mirror of 
the heliostat and the achromatic condenser. Dr. 
’ Woodward’s regulator, which is recommended to 
those who desire to provide themselves with ap- 
paratus of this kind, is provided with an eight-inch 
lens carried at the outer extremity of a tube pass- 
ing through the frame or plate of the regulator, 
while the inner extremity carries a metal pocket 
into which the ammonio-sulphate cell is intro- 
duced. 
