ECLIPSES, PHOTOGRAPHING The portrayal of the varying phases of an eclipse, whether of the sun or moon, is always of interest to the photographer. The exact times at which these phenomena will occur can always be ascertained a considerable time in advance from the almanacs, thus enabling the photo grapher to prepare any special apparatus he may desire to fit up for the occasion.
Dealing first with eclipses of the moon, the chief interest in these lies in the gradual change in shape as the moon traverses the earth's shadow. When fully eclipsed the moon may be either quite invisible, or showing of a more or less bright coppery colour. The most satis factory manner of obtaining a picture of the eclipse with an ordinary camera is to set up the camera on its tripod and focus sharply ; then swing until the moon is near one end of the ground glass. The end chosen should be that from which the moon's image will travel in its motion across the sky owing to the earth's rotation. This is always from east to west, so that, remembering that everything is inverted in the camera, the first exposure should be arranged near the right-hand side of the plate if the observer is in the northern hemisphere. Loai the dark-slide with isochromatic plates of medium rapidity; draw the slide, and expose with the cap. Give about a quarter of a second exposure. After making one exposure, leave the apparatus as it stands, with slide still drawn, for say five minutes. During this interval the moon's image will have travelled towards the left on the plate, and another similar exposure may then be given. Repeat this procedure at intervals of about five minutes until the eclipse is over. On developing, which will be exactly like the development of an ordinary subject and needs no special description, a series of pictures of the moon will be found extending across the plate, and of varying shapes, from full circle, through crescents to the dusky total eclipse. A pleasing variation to the above procedure may be made if the observer has a telephoto equip ment, as he can with it obtain larger pictures of the moon's phases, showing more details of the surface structure. It will not be possible, how ever, to obtain the whole series of pictures illus trating the progress of the eclipse on a single plate owing to the increased magnification. If the series is required, then several loaded plate holders must be got ready to hand and the plates changed as found most convenient.
With eclipses of the sun, the problem is some what different, chiefly on account of the very great actinic power of even a small section of the uneclipsed solar disc. If the eclipse is only partial, the arrangement described above for the lunar eclipse may be repeated except in regard to the exposure. This must be rendered as short as possible. If a focal plane shutter is to be employed, the slit in the blind must be made as narrow as possible, and the tension run up to the maximum. Should a diaphragm or cap
shutter only be available, then it will be advis able to stop down the lens to the smallest aper ture possible and also use the shutter at its greatest speed.
The greatest interest, however, becomes cen tred in the solar eclipse which becomes total, the dark moon blocking out all the usual brilliant white disc. Then it is that one is able to see that wonderful appendage to the sun, the corona, and the ring of ruddy atmosphere, the chromo sphere. On a small scale these features may be photographically recorded by means of cap exposures, but they will have to be fairly rapid, say a quarter of a second, owing to the rapid motion of the moon. With a small mechanical stand having provision for driving the camera at the same rate as the drift of the moon much longer exposures may be given, and the coronal extensions portrayed to a greater distance from the moon's limb. Owing to the special nature of the coronal light isochromatic plates should be employed for this class of work, and, preferably, the most rapid variety available. For recording solar eclipses on a very large scale, special cameras with lenses of great focal length have been at various times employed. For example, cameras with lenses of 4o feet focal length were used by an American party under Professor Campbell in India. The lens was situated at the narrow end of a tube, propped up on the top of a wooden skeleton tower. The plate holder was in a canvas-covered portion near the ground, and in one case the moon's motion was counter-balanced by moving the plate-holder by a simple form of clepsydra.
One of the most interesting and instructive applications of photography to a total solar eclipse consists in the use of the prismatic camera. In its simplest form this is the ordinary camera with a simple prism adjusted outside the lens in such a position that the light from the eclipsed sun must pass through both prism and lens before it reaches the photographic plate in the camera. By this means is obtained a series of pictures of the sun's surroundings, instead of a single one as before. Every different substance present in the sun's atmosphere will show as a different ring or series of rings, and it is by a study of such photographs—or spectrograms, as they are technically called—that astronomers have been able to learn what substances are present in the solar atmosphere. In a particu lar example of the prismatic camera, the lens aperture is 6 inches, and the focal length of the lens is 7i feet. There is a single large prism of 45° refracting angle rigidly attached outside the objective, and the whole instrument is fixed to a strong equatorial mounting driven by a delicately adjusted clockwork movement, so that exposures of any desired duration can be given.