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DRY PLATES, FILMS AND PAPERS 167
much larger proportion of yellow rays than when it is higher in the sky, orthochromatic plates can be used without a screen, and the pictures obtained will be distinctly better than those got with ordinary plates. On the other hand, for ordinary purposes no benefit is derived from the use of such plates unless the screen is employed.
The employment of screens is, however, quite a simple matter, for they can be obtained so as to fit on the front of the lens, and the cap of the screen can then be used if desired to time the exposure, instead of the ordinary lens cap. When not in use, colour screens should be kept in a light-tight case, as unnecessary exposure to light only tends to alter the light value of the screen.
The result of using orthochromatic plates with suitable colour screens is well illustrated in figs. 78, 79, 80, and 81.
The importance of the orthochromatic plate and colourfilter in astronomical work is clearly shown by the following statement by R. J. Wallace : 1 —
" The new era in photographic science, opened by the introduction of the (so-called) isochromatic plate and its accompanying colour filter, was pregnant with significance to astronomers in general throughout the world, for that hitherto while the application of photography to the recording of results could be attained only by the possession of an expensive correcting lens, the simple combination of a colour filter and isochromatic plate not only fulfilled all requirements, but did so more perfectly/'
Care must be taken not to over-develop orthochromatic plates, for then the high lights will be partly obliterated in the attempt to bring out the details in the shadows.
Now that plates are being used which are sensitive to red light, the ordinary dark-room lamp or lantern which supplies red fight must not be used when developing the plates, in fact, as little light of any kind should be used as it is possible to work with, and then it ought to be of
1 Astro physical Journal, xxvii. 2, 1908.