SPECTROSCOPE (from Lat. spectrum, ap pearance, image, apparition + Gk. Cf KOTE7r, akopcin, to view). An instrument designed to investigate the nature of the radiations emitted by various sources of 'light,' it being understood that this term includes all waves in the ether, although only those within certain narrow limits of wave-length affect the sense of sight. It is shown in the article on SPECTROSCOPY that radia tions are being sent off in the ether from all natural bodies, and that these are in the form of waves of different wavelength. When these waves pass through a spectroscope they are dis persed in such a way that the waves of certain wave-length are brought to focus at a point dif ferent from that to which waves of a different wave-length are brought. In this way the radia tions from any source are analyzed and spread out in what is called a 'spectrum.' The essential features of a spectroscope are then: first, a slit, or extremely small source of radiation; second, some of producing dispersion; third, a lens or other means of focusing the radiations at the eyepiece of a telescope, upon a screen, or upon some suitable recording instrument. If the radia tions are of such a nature as to affect a photo graphic plate, that is, if they are in the ultra violet or in the visible portion of the spectrum, methods of photography may be used in connec tion with the spectroscope. (See SPECTROGRAPH.) If the radiations are in the infra-red, that is, if the wave-lengths are so long that they do not af fect the sense of sight, instruments must be used which are sensitive to such radiations; for in stance, a thermometer. bolometer, radio-micro meter, radiometer, or other heat-registering de vice. It has been found that if a plate of glass covered with some phosphorescent substance, such as MtImain's paint, is exposed to light and then carried into a darkened room, it will con tinue to be luminous, for seine time: but, if ex posed in a spectroscope to infra-red radiations, the phosphorescence at those points reached by the radiations is destroyed. This furnishes a method, therefore, for the study of these long waves.
As ordinarily constructed, a spectroscope has the same general appearance as a spectrometer (q.v.). There is, however, in addition, in case prisms are used to produce dispersion, some aux iliary apparatus for the purpose of enabling the observer to record numerically the positions oc cupied by the waves which he is observing, in comparison with other waves. One method which
is in common use is to attach to the instrument a tube containing at one end a transparent scale, and at its other end a lens, the tube itself being so placed that when the scale is illuminated by a lamp the waves proceeding from it fall upon the last, face of the last prism and are reflected in such a manner as to be brought to focus in the same plane as are the radiations under investiga tion. By this means there is produced across the spectrum a series of lines regularly spaced and numbered, and the position of any radiation can he recorded.
Various means are used to produce dispersion, but the two methods most generally adopted are (1) to interpose a prism or train of prisms between the collimator and telescope. or (2) to allow the light from the collimator to fall upon a diffraction grating (q.v.). The dispersing action of a prism and of a grating has been explained before (see DIFFRACTION AND DIFFRACTION GRATINGS) ; but there are several important differences be tween the spectra obtained by these two instru ments. Prismatic spectra are said to be 'irration al' because there is no simple relation between the material and shape of the prism and the dis persion produced by it ; and, further, because prisms of the same shape and size produce quite different spectra, in the sense that the relative deviations of the same waves differ widely when different prisms are used. See DISPERSION.
The spectra produced by plane gratings, on the other band, obey a definite law, there being an extremely simple relation between the constants of the grating and the deviation of a given train of waves, which is independent of the material of the grating, and which enables one by simple means to measure the wavelength of the radia tions being studied. If a concave grating is used in place of a plane one, as was first done by Powland in 1832, it is not necessary to have lenses in the spectroscope, the essential parts of the instrument then being simply a slit. a grat ing, and some receiving apparatus, such as a pho tographic plate. There is a further advantage in the use of a concave grating (which may also be obtained with a plane grating if suitably ad justed) if this instrument is used in the standard manner, in the fact that the spectra produced are of such a kind that the distances along the photographic plate are proportional to differences in wave-length of the waves which are thus re corded. A spectrum of this kind is said to be `normal.'