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ASTRONOMICAL PHOTOGRAPHY 309
The plates used by Slipher x for this purpose were "" Seed 23," which he sensitized to the lower spectrum by bathing them for 3 or 4 minutes in a bath made up as follows : —
Water, 8 ounces.
Dicyanin, 45 minims.
Pinaverdol, 75 minims.
Pinacyanol, 25 minims.
Ammonia, 120 minims.
After removing the plates from this bath they are washed in water, or rinsed in alcohol and dried in a current of warm air. He finds that this combination of dyes produces a plate which is sensitive in a fairly uniform manner to the prismatic solar spectrum down to A7000, where it begins to weaken. It is just sensitive enough in the region about line A to record that line.
The importance of this method of sensitizing will be understood when it is mentioned that almost all the dark lines observed in the solar spectrum below line C are due to absorption in the earth's atmosphere.
Previous observations which have been made with a view to settling the question as to the presence of water vapour in the atmosphere of Mars have been greatly hampered by the fact that they have been of necessity confined to the rainband which is visible in the spectrum round about the D line ; but this plate gives the " a " band, which is situated midway between the oxygen bands A and B.
This double band is due to the absorption of light by the water vapour in the air. It is by far the strongest of the group of lines due to water vapour, being visible when there is but a very small amount of moisture in the path of the light, and is even conspicuous when the rainband is not visible.
1 Astrophysical Journal, Dec. 1908.