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ASTRONOMICAL PHOTOGRAPHY 319
to point A, and then exposure to the faint spectrum can be made at that period when the increase in density takes place most rapidly.
To find out just what is the best preliminary exposure to give, turn down an ordinary gas jet until its yellow tip is only three or four millimetres high ; hold the plate a certain distance from it covered with a sheet of black paper. If now the black paper is gently and gradually drawn aside so as to expose strips in which the exposure will vary by two seconds, the results can be compared on development. Wood finds that glycin is the best developer to use, and he prefers a rather strong developer which he uses for some fifteen to twenty minutes.
The best results are obtained by the exposure which produces a faint image ; this is obtained in four seconds, Avith the small gas jet previously mentioned at a distance of about two metres, but of course this will depend upon the plate used. According to Wood, it is immaterial whether the exposure to the faint source of light is made before or after exposure to the weak stellar spectrum or nebula.
On the other hand, R. J. Wallace and H. B. Lemon1 contend that no practical advantage is gained by either a preliminary or supplementary exposure in so far as stellar photography is concerned.
Colour of Light from Stars. — Schwarzschild has suggested that the difference between the visual magnitude of a star and that obtained from ordinary photographic plates should give an accurate measure of the star's colour. He named this difference the " Farbentbnung."
The method of determining the colours of the stars by the eye has proved very unsatisfactory. Parkhurst and Jordan 2 have now made use of photographic means to obtain the visual magnitude of stars.
1 Astrophysical Journal, xxix. 2. 2 Ibid., xxvii, 3, 190S.