PHOTOGRAPHIC SPECTRA Spectroscopic work of precision is now almost invariably carried on by photographic methods. A photographic record of a spectrum has many obvious advantages ; it can be examined as often as desired by any number of persons without the need for repeating the experimental arrangements, and it can be more conveniently and more accurately measured. Moreover, with appropriate instruments, the invisible ultra-violet part of the spectrum can be included in the photographic record, and by the use of specially prepared photographic plates a considerable portion of the near infra-red region, extending as far as X io,000k or can also be photographed. The additional region in the
violet thus opened up is of considerable importance from the point of view of spectrum analysis, and both regions are of great sig nificance in connection with investigations of the structure of spectra. The photographic plate has a further important advan tage arising from the power of integrating successive impressions, whereas the eye can only take account of momentary effects on the retina; spectra which are too faint for satisfactory direct observation may thus be made available for investigation by photographs taken with long exposures.
In their optical arrangements, spectrographs are similar to the spectroscopes intended for visual observations. It is, in fact, only necessary to replace the observing telescope by a camera provided with a suitable object glass as shown in fig. 8. If the object glass of the telescope itself were utilized and the spectrum could be photographed on a perfectly trans parent plate, it will be clear that when viewed with the eyepiece as magnifier, the spectrum would be identical with that directly observed, apart from the absence of colour. Photographic plates, however, have a granular structure and lines which might be just separated visually would not appear separated on the plate. Thus, to obtain a photographic resolution equivalent to the visual, it is necessary to use a camera lens of considerably greater focal length than that of the ordinary telescopic objective, so that the spectrum may be magnified before it reaches the photographic plate. It is desirable, though not essential, that the collimating lens should have a focal length not very different from that of the camera lens, and with the ordinary construction such instru ments are apt to be unwieldy. Long focus spectrographs are
accordingly now most frequently built up on the plan suggested by Littrow. In this form the same lens serves as collimator and camera lens, and light passing through the prism is returned along the same path either by a plane mirror behind the prism, or by a half-prism silvered on the back, as shown in fig. 9. In such an instrument the slit, S, is placed at the side and light entering it is directed along the axis of the lens by a small reflecting prism p, and it is easy to arrange matters so that the spectrum enters a narrow aperture in the camera at P just above the level of the reflecting prism. The Littrow arrangement has the advantage that each dispersing prism acts as two prisms, and in addition, the instrument may be made of great rigidity.
Since there is nothing on a photographic plate to indicate the wave-lengths of any of the lines, except that many lines can be identified by their peculiar groupings, it is usually necessary to photograph a known spectrum, which has already been accurately measured, in juxtaposition with that under observation.
Since glass is not transparent to the ultra violet and infra-red rays, or only to a small extent, prisms and lenses made of other substances are requisite for the exploration of these parts of the spectrum. Prisms of rock salt, which are largely used for the infra-red, will transmit as far as wave-length I 5o,000A, or i5/2, whilst a fluorite prism will transmit as far as 9A, and one of sylvine as far as 23/2. To reduce loss of intensity by absorption, concave mirrors are used in place of lenses for collimation and image formation. A delicate thermopile, which is connected with a highly sensitive galvanometer, reveals the pres ence or absence of spectrum lines as the spectrum is moved across it by rotating the prism. Readings on the scale of the prism table are then convertible to wave-lengths with the aid of stand ards which have been determined by the use of gratings. By the use of a large concave grating in place of a prism, F. Paschen has measured many spectra as far as 3.1A, With the aid of Eastman Kodak's neocyanine and kryptocyanine plates, the infra-red spec trum can now be directly photographed as far as IA.