Following the decision to set up Photographic and Spectroscopic Department, a spectroscope was ordered from Browning. Delivered to the Observatory on 2 May 1874, it was ‘specially adapted to sweeping around the Sun’s limb, with a view to mapping out the prominences’. It was also available for work on Stars and Nebulae, the dispersive power being readily varied. Browning also made an induction coil, capable of giving a six-inch spark. Other equipment obtained for use with the spectroscope were a bichromate of potash battery and a Bunsen battery, each of three cells (ADM 190/4/384). Designed for use on the 12.8-inch Merz refractor, the browning spectroscope was used until July 1877 when it was superseded by Christie’s direct vision "half-prism" spectroscope. Recorded in the 1926 inventory (RGO39/05/103) as being in the South Building (Lower Museum, Case F), its current whereabouts is unknown.
The text below has been extracted from the introduction to the 1877 volume of Greenwich Observations. To read it in its original format, click here.
‘The form of this spectroscope (made by Browning) has been arranged so to give great facility for varying the dispersive power, and its attachment to the Great Equatoreal has been planned with special reference to the examination of the prominences round the Sun’s limb by an excentric rotation of the spectroscope. The light of the object viewed passes through a narrow slit at the focus of a collimator; and after divergence upon the collimator object-glass and refraction by it, falls as a parallel-pencil upon the first prism. The full train of prisms consists of one “half-prism,” four whole prisms, and another “half-prism” silvered on the back. The prisms are compound, being each composed of a prism of flint with an angle of about 94º, and two thin prisms of crown cemented to the flint. The “half-prisms” are virtually formed by cutting a whole compound prism in halves by a plane perpendicular to the base, The “half-prism” silvered on the back or perpendicular face can be inserted in any part of the train, so as to give a dispersive power of two, four, six, eight, or ten prisms, the rays being reflected directly hook at the same level, and passing twice through the train and collimator. A spectrum is thus formed in the plane of the slit, and is viewed by mean of a diagonal prism with eye-piece within the collimator-tube, which ill placed just clear of the course of the cone of incident rays, and intercepts the rays for one-half of the field on their return after passing twice through the train of prisms, the collimator itself being thus used as viewing telescope.
A lower dispersive power is given by inserting a plane reflector immediately behind the first half-prism; the rays thus pass twice through this half-prism, and a dispersive power of one prism is obtained. In another form of the instrument, a viewing telescope can be inserted in any part of the train, receiving the rays after they have passed once through one half-prism, or one half-prism and one, two, or three whole prisms.
The prisms are each 2 inches high and 1.6 inches broad in cross-section, the thickness being 3.4 inches at the base and 2.1 inches at the refracting edge. The collimator has an aperture of 1.6 inches and a focal length of 7 inches, but this is virtually increased to 24 inches by a concave lens within the principal focus of the object-glass. From the circumstance that the rays are reflected directly back to form the spectrum, some trouble was occasioned by false light reflected from the surfaces of the lenses in the collimator, the spectrum with the full power of ten prisms being greatly enfeebled by absorption in passing through nearly three foot of glass. To obviate this inconvenience two concave lenses, placed side by side, with all intervening diaphragm, were substituted on 1874, August 24, for the single concave lens in the collimator-telescope, so that the rays which returned through the collimator to the eye passed through a different concave lens from that on which the incident pencil fell. As the results for displacement of lines in stellar spectra appeared to be affected by this arrangement, it was abandoned and the original plan reverted to on 1875, March 4. For comparison with the spectrum of hydrogen or other chemical element, a vacuum tube was, in the earlier observations, placed centrally within the tube of the Equatoreal, at a distance of either two or four feet from the slit; but as systematic errors appeared to be introduced by the different conditions of the pencils from the star and from the vacuum tube, another method has been used from 1875, May 31, in which an image of the vacuum tube (or electrodes) is formed on the slit immediately above and below that of the star, by means of two comparison prisms in connexion with a collimating lens, so that the cone of rays from the comparison-light, as well as that from the star, fills the whole of the object-glass of the collimator. Both the collimator-telescope and the viewing telescope are provided with micrometers; larger differences of wave-length are inferred from the readings of a scale and tangent-screw which show the space through which the train of prisms has beeon moved.
This spectroscope was used till 1877, July, when it was superseded by Mr. Christie’s direct-vision or "half-prism" spectroscope constructed by Mr. Hilger.’
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