SPECTROGRAPHY (Fr., Spectrophotographie; Ger., Spektrophotographie) Any spectroscope may be converted into a spectrograph or camera for photographing the spectrum by attaching to the telescope a camera instead of an eyepiece. The simplest form is that in which a direct-vision spectroscope is used, and it may be either an ordinary camera or merely an oblong box to the front of which the spectroscope is attached, whilst the back car ries the dark-slide for the plate. The usual eye piece lens must be removed, and care should be taken that no part of the mount cuts off the spec trum. No lens need be used, and the length of the spectrum is then solely dependent on the extension of the camera. If a lens is used, then the lines will be sharper. A shows the simple box form without lens ; F is the box, D the direct vision prism, B the slit, A the condenser, and E the milled head for focusing the collimator lens.
B shows the form with lens D behind the prisms ; D is an outer tube that prevents stray light having access to the camera, K is the groove for the dark-slide, and E a central screw, on which the back can be swung to any angle and then fixed by the screw F ; b is a short length of bellows.
When using the direct-vision spectroscope, the line that passes straight through is generally the D line ; therefore the spectroscope must be placed above the centre of the plate, other wise the violet end will not be included. With the spectrograph without lens, the length and width of the spectrum are dependent solely on the extension of the camera.
In the ordinary single or two-prism spectro scope the telescope should be entirely replaced by a light camera, and it is advisable to cover over the prism table and the lenses so that no light has access to the plate but through the slit ; otherwise general fogging of the plate will ensue.
It is convenient to take a series of negatives on one plate, and this can be done by small sliding shutters in front of the slit, but more conveniently by providing a slit, say, of one inch width, in the back of the camera and arranging for the dark-slide to be shifted in its grooves so that three or four contiguous spectra may be obtained by merely shifting the plate. With
all prismatic spectrographs it is essential to arrange for the swinging of the plane of the plate ; that is, for putting the violet end nearer the lens than the red. If the spectroscope is always to be used with the same prisms and lenses, then this can be done once for all; other wise slots and screws must be provided as shown in diagram B.
One great disadvantage of the prismatic spectrograph is the uneven dispersion, the red being cramped together and the blue and violet more extended. On the other hand, this may be an advantage in photographing very faint absorptions or sensitising action in the red if mere qualitative and not quantitative results are desired. This unequal dispersion is well shown in C and D ; the former represents the spectrum produced by a diffraction grating and the latter a spectrum of equal length and disper sion produced by a prism.
With all glass there is more or less absorption of the ultra-violet, and with heavy flint glasses with great dispersion this may even extend into the visible violet. Quartz, calcite, and fluorite are very transparent to this region, and must be used both for prisms and lenses if photo graphy of the ultra-violet is to be attempted. Undoubtedly the concave grating which requires no lens is most useful for this region. Practic ally, ordinary glass may be considered not to transmit beyond A 3,40o when a thickness of about I in. is used. Another disadvantage with the prismatic spectrograph is that the lines are curved with the convex side towards the red, due to the fact that only those rays which pass through the centre of the slit can pass through a plane perpendicular to the refracting edge of the prism, which is usually called the principal plane.