The Photographing of Patients

Photographic Telescopes.

In the ordinary telescopic objec tive the lenses are so shaped that the rays of the spectrum about the yellow are brought as nearly as possible to pass through the same point as focus, for these are the rays to which the eye is sensitive, but for photography it is the blue and violet rays that must be brought to a focus, for it is these that most affect the plate. An objective of two lenses may be made to do this, but necessity has led to the making of photo-visual objectives of three lenses that are perfectly or nearly achromatic, and focus all the rays (v. p. 836, Franklin-Adams). It is possible to use a visual telescope for photography by adding a correcting lens, or another scheme for the purpose, adopted for the 4o in. refractor of the Yerkes observatory about 1900, is to take photographs through a yellow colour screen or filter placed almost in contact with a yellow-sensitive (Cramer instantaneous achromatic) photographic plate. The yellow screen transmits freely to the plate the sharp yellow image produced by the visual objective and excludes the blue and other wave-lengths which would otherwise destroy the sharpness of the photographed images. Numerous experiments have since been made to find the most suitable filters and plates for special purposes.

Since the mirror brings all rays to the same focus the reflec tor has an advantage over the refractor. The field of view of the reflector, however, is relatively small. For a focal ratio of 1 to 5 the aberrations of the reflector are disturbing at distances from the optical axis exceeding 12' to 15', and there is considerable loss in limiting magnitude for even smaller distances, whereas a properly designed refractor affords excellent definition over a field several degrees in diameter. Measures with reflectors are therefore limited to double stars or satellites of planets, or to the determination of proper motions and parallaxes, etc., while the refractor finds advan tageous application to general charting and cataloguing purposes.

That the optical possibilities are by no means exhausted is indi cated by the quadruplet lens recently designed by F. E. Ross which, with an aperture of 3 inches and a focal ratio of 1 to 7, gives excellent definition over a field of 25°, and in two hours exposure reaches a limiting magnitude of 15.4.

The choice of aperture and focal ratio for a photographic tele scope depends on the purpose for which the instrument is to be used and is determined by the same principles that apply in ordinary photographic practice. Increased focal length means in creased scale or magnification in the photograph. For a given focal length, increased aperture means greater brightness in the optical image, and hence greater photographic efficiency, which may be utilized either to reduce the exposure time or to photograph fainter objects than would otherwise be possible. For extended objects like planets or nebulae, the brightness of the image is proportional to The photographic efficiency is independent of the linear dimensions, and the chief advantage of a large instrument is that of magnification. At the same time, the instrument must not be

too small, otherwise the power of resolution or minimum separa tion which varies inversely as the aperture, will suffer. For point sources, such as stars, the diameter of the central diffraction disc of the image decreases as the aperture increases. This introduces an additional factor whence the brightness is proportional to For a given focal ratio there is in this case an important gain in efficiency with increasing aperture, although it is less than the elementary theory would indicate. Aberrations and absorption and reflection losses in the objective, imperfections in the optical surfaces, and tremor of the diffraction disc, caused by atmospheric disturbances, all operate in diminishing the theoretical gain.

The application of these principles to different classes of obser vational problems is well illustrated by some of the instruments of the Mount Wilson Observatory. For observations of the sun, both direct and spectroscopic, high magnification is desirable. Since there is an abundance of light, the focal ratio need not be large ; and, further, since a moderate aperture affords adequate resolution for photographic purposes, the dimensions chosen for the larger Tower telescope are a foot, f =15o feet, with a ratio of 1 to 15o. For the efficient photography of very faint stars, both the focal ratio and the linear dimensions should be as large as possible. Hence in the Hooker reflector we find a = ioo inches, f = Soo inches, with a ratio of to 5. Under favourable atmospheric conditions, the theoretical gain over the 6o-inch reflector, of the same focal ratio, in photographing stars is very nearly attained. Aside from differences of scale, these two instruments are of equal efficiency in photographing nebulae. For planets, however, the 1 oo-in. is practically the more effective. For the same magnification (enlarg ing lenses are commonly used in planetary photography) this in strument gives a brighter image than the 6o-inch reflector. This permits the use of shorter exposures, which avoids in part the blurring produced by atmospheric disturbances. The useful field, the same for both reflectors, is necessarily small (v. p. 837). For photographs covering a large field, a triplet refractor is available (a = io inches, f =45 inches, a/f = I/4.5). Because of the focal length, the scale is small. Otherwise, this instrument is of about the same efficiency for nebulae as the large reflectors, and because of the large field, is very advantageous for photographing widely extended masses of nebulosity. Its large focal ratio makes it an efficient instrument for stars, although the limiting magnitude attainable falls far short of that reached by the reflectors.