On the aqueous lines of the solar spectrum / by Josiah P. Cooke.

  • Cooke, Josiah P., Jr. (Josiah Parsons), 1827-1894.
Date:
[1866?]
Catalogue details

Licence: Public Domain Mark

Credit: On the aqueous lines of the solar spectrum / by Josiah P. Cooke. Source: Wellcome Collection.

Provider: This material has been provided by King’s College London. The original may be consulted at King’s College London.

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    not appear until the air is very moist; and even when it contains 6.57 grains of vapor per cubic foot, they are still very faint. Under yet more unusual atmospheric conditions they will undoubtedly become more intense, and we shall then probably be able to completely lesolve the nebulous band and count the lines of which it consists. It is hardly necessary to repeat, thSt the examples here given are selected from a large number of observations. During the cold dry weather of winter the appearance of the D line is uniformly as shown in Fig. 1, the line ^ only occasionally appearing when the atmosphere becomes more moist. During the warm weather of summer, when the absolute amount of moisture in the air is in almost all cases greater than in winter, the appearance of the D line is as uniformly that shown in Fig. 3. It is only very rarely in the dry climate of New England, even during the summer, that all the lines shown in Fig. 4 are visible; and, as already stated, I never saw them before so sharply defined as on the 17th of November last. Several conditions must evidently concur in order that the aqueous lines should be developed in their greatest intensity. In the first place, the air must be charged with vapor not only near the surface of the •earth, but also through a great height of the atmosphere. Local causes might greatly increase the amount of moisture in the lower strata of the atmosphere, and affect powerfully the hygrometer, which would not, to the same extent at least, influence the indications of the spectro- scope. In the second place, other things being equal, the intensity of the aqueous lines must be strengthened by increasing the length of the path of the sun’s rays through the atmosphere, and this is the longer the lower the altitude of the sun. But then, again, the intensity of the light has such an important influence on the definition of the lines, and the slightest haze in the atmosphere so greatly impairs their dis- tinctness, that I have generally found that the aqueous lines are seen best when the sun is near the meridian. Hence, with an equal amount of moisture in the atmosphere, the late autumn may be a more favor- able season for seeing the aqueous lines than the summer; for then not only must the solar rays, when most brilliant at noon, traverse a greater extent of air, but moreover the atmosphere at this time is usually clearer, and the reflected beam of light which enters the spec- ti-oscope is at times more brilliant than when the sun attains a higher elevation and the light is reflected under less favorable conditions. In the examples cited above, the comparisons were made under as