SPECTROHELIOGRAPH  , an

instrument for photographing the sun with monochromatic light . In its simplest form it consists of a direct-vision spectroscope, having an adjustable slit (called " camera slit "), instead of an eyepiece, in the focal plane of the observing telescope . This slit is set in such a position as to transmit a single line of the spectrum, e.g. the K line of calcium . Suppose a fixed image of the sun to be formed on the collimator slit of this spectroscope, and a photographic plate, with its plane parallel to the plane of the solar image, to be mounted almost in contact with the camera slit . The spectroscope is then moved parallel to itself, admitting to the collimator slit light from all parts of the sun's disk . Thus a monochromatic image of the sun, formed of a great number of successive images of the spectral line employed, will be built up on the plate . As the only light permitted to reach the plate is that of the calciumline, the resulting image will represent the distribution of calcium vapour in the sun's atmosphere . The calcium clouds or flocculi thus recorded are invisible to the eye, and are not shown on direct solar photographs taken in the ordinary way . The calcium flocculi, on account of the brilliant reversals of the H and K lines to which they give rise, and the protection to the plate afforded by the diffuse dark bands in which these bright lines occur, are easily photographed with a spectroheliograph of low dispersion . In the case of narrower lines, however, higher dispersion is required to prevent the light of the continuous spectrum on either side of the dark line from blotting out the monochromatic image . A spectroheliograph which gives excellent results with the lines of calcium, hydrogen and iron is shown in the figure . This instrument, used since 1905 in conjunction with the Snow (horizontal) telescope of the Mount Wilson Solar Observatory, was constructed in the observatory instrument shop in Pasadena .

It consists of a heavy

cast-iron platform (a) mounted on four steel balls (b) which run in V guides of hardened steel . Most of the weight of the instrument is floated on mercury contained in three troughs (c, c, c) which form part of the cast-iron base . The platform carries the two slits, the collimator and camera objectives and the prism-train . An image of the sun, about 6.7 in. in diameter, is formed by the Snow telescope on the collimator slit (d) . This slit is long enough (81 in.) to extend entirely across the solar image and across such prominences of ordinary height as may happen to lie at the extremities of a vertical diameter . After passing through the slit the diverging rays fall upon the 8 in. collimator objective (e), which is constructed in the manner of a portrait lens in order to give a sharp field of sufficient diameter to include the entire solar image . In the Snow telescope the ratio of aperture to focal length is 1 :3o . Hence light from any point on the slit will fill a circle about 2 in. in diameter on the collimator objective, as its focal length is 6o in . Since the diameter of the solar image is 6.7 in. there is a slight, but inappreciable loss of light from points in the image at the extremities of a vertical diameter . The rays, rendered parallel by the collimator objective, meet a plane mirror (f) of silvered glass, which reflects them to the prisms (g, g') . These are of dense flint-glass (Schott 0.102), and each has a refracting angle of 63° 29' . Their width and height are sufficient to transmit (at the position of minimum deviation) the entire beam received from the collimator .

After being deviated 18o° from the

original direction, the dispersed rays fall on the camera objective (h), which is exactly similar to the collimator objective . This forms an image of the solar spectrum in its focal plane on the camera slit (i) . Beyond the camera slit, and almost in contact with it, the photographic plate-carrier (j) is mounted on a fixed support . In order to bring a spectral line upon the camera slit, the slit is widely opened and the plane mirror (f) rotated until the line is seen . A cross-hair, in the focal plane of an eyepiece, is then moved horizontally until it coincides with the line in question . The slit is narrowed down to the desired width, and moved as a whole by a micrometer screw, until it coincides with the cross-hair . The eyepiece is removed and the photographic plate (k) placed in position . An electric motor, belted to a screw (1 or 1') connected with the spectroheliograph, is then started.' The screw moves the spectroheliograph at a perfectly uniform rate across the fixed solar image . Thus a monochromatic image of the sun is built up on the fixed photographic plate . The spectroheliograph, originally designed for photographing the solar prominences, disclosed in its first application at the Kenwood Observatory (Chicago, 1892) a new and unexplored region of the sun's atmosphere . Photographs of the solar disk, taken with the H or K line, show extensive luminous clouds (flocculi) of calcium vapour, vastly greater in area than the sun-spots . By setting the camera slit so as to admit to the photographic plate the light of the denser calcium vapour, which lies at low levels, or that of the rarer vapour at high levels, the phenomena of various superposed regions of the atmosphere can be recorded .

The

lower and denser vapour appears as bright clouds, but the cooler vapour, at higher levels, absorbs the light from below and thus gives rise to dark clouds . The first photographs of the sun in hydrogen light were made with the spectroheliograph in 1903 . These reveal dark hydrogen flocculi, which appear to lie at a level above that of the bright calcium flocculi . They also show less extensive bright flocculi, usually in the immediate neighbourhood of sunspots, and frequently eruptive in character . These rise 'Two screws, of different pitch, are provided, to give different speeds . from a low level, and sometimes reach considerable elevations in the form of eruptive prominences . In such an exploration of the sun's atmosphere it might be anticipated that definite currents, or some evidences of atmospheric circulation analogous to those familiar in terrestrial meteorology, would be discovered . Neither the forms nor the motions of the calcium flocculi revealed the existence of such b The Five-foot Spectroheliograph of the Mount Wilson Solar Observatory (camera lens, camera slit and plate carrier in section) . currents, but in the higher region shown by the hydrogen photographs the distribution of the dark flocculi suggested the operation of definite forces, though their nature remained obscure until the spring of 1908 . At that time monochromatic photographs of the sun were first made on Mount Wilson with the red (Ha) line of hydrogen, previous hydrogen photographs having been taken with H/3, Hy or HS in the blue or violet . On account of the relatively great strength of Ha at a consider-able distance from the photosphere, the new photographs recorded flocculi at high levels previously unexplored . The forms of these flocculi show that all sun-spots are vortical in nature, and are probably analogous to terrestrial cyclones or tornadoes .

Most of the solar vortices indicate clockwise rotation in the

southern hemisphere and counter-clockwise rotation in the northern, as in the case of terrestrial cyclones . But frequent exceptions have been observed in which the direction of rotation is reversed . The study of these vortices has led to the discovery of a magnetic field in sun-spots, apparently caused by electric convection in the vortices . It is evident that by the use of a spectroheliograph of sufficiently high dispersion, photographs may be taken of vapours in the sun represented by lines narrower than those of calcium and hydrogen . Such work has been in progress both at Mount Wilson and at Meudon, and the erection of a spectroheliograph of 75 ft. focal length on Mount Wilson was at the end of 1908 contemplated for an early date . Descriptions of spectroheliographs by Hale, Deslandres, Newall and others, may be found in various papers in Astronomy and Astrophysics, Astrophysical Journal, Comptes rendus, Bulletin astronomique, and other periodicals . (G . E .