such changes it would be practicable to take several negatives at about the same hour of each day, an actinometer being used, and the exposures varied in accordance with its indications. The operations might thus be omitted during bad weather, an additional number of photographs being taken on succeeding fine days. Some very pleasing views for the cinematograph could without doubt be obtained by this mode of procedure.
Turning now from the earth to the heavens, we shall see that similar methods are applicable to the most prominent of celestial bodies—viz., the sun. The photographic art has long since been applied with conspicuous success to the glowing solar disk, with its dark spots and brighter patches or "faculæ"; and such photographs are now taken from day to day at leading observatories in various parts of the globe. During recent years, moreover, astronomers have contrived to photograph, under ordinary conditions, the surroundings of the great luminary—including the chromosphere and prominences, but excepting the corona, which can not as yet be studied in the absence of an eclipse.
I shall not attempt to describe the many interesting features shown in such photographs; nor is it necessary in this place to indicate the precise means whereby solar picture films can be produced. The chief point to be noted is that changes—often of a rapid and striking character[1]—are continually occurring both in the sun's photosphere and its gaseous surroundings. The cinematograph will enable us to actually see such changes taking place; and it may be possible in this way to obtain new light on certain fascinating, though recondite, problems presented by the sun, while the complex solar movements may in any case be pictured in a manner that can not fail to prove deeply interesting and instructive.
Although the common motion views are often described as realistic, there are two respects in which they fail to correctly represent the original scenes. Not only do they lack the charm of color (which adds so much to the variety and interest of ordinary scenes), but the effect of solidity, due to our binocular vision, is also absent. As regards the reproduction of color, we shall have to rest content—at least for some time to come—with monochromic views of ordinary moving objects. Instantaneous photography in colors is not yet possible, nor is it likely to be achieved in the near future. In the color process of M. Lippmann, for example, it is necessary to expose the sensitive plates for relatively long periods of time. It may be questioned, also, whether such photographs could be successfully produced upon celluloid films. And even if these objections were re-
- ↑ We are here concerned with movements which, though invisible to the observer's eye, take place in reality with very great rapidity.
