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account of repeated deviation bringing the rays to the point from which they started, various arrangements are used to elevate the rays when they reach that point, and either re-transmit them through the same or another series of prisms. In spectroscopes of this construction we get both dispersion and deviation: in order to secure direct vision the arrangement of the prisms must be different, and such an instrument is accordingly called a direct vision spectroscope. The direct vision spectroscope is generally constructed on the principle that a flint glass prism will give a much longer spectrum than a crown glass prism whilst causing an equal deviation of light. If we look through a single flint prism we shall see an object considerably displaced; but if we now put in the opposite direction a crown prism of sufficient power to correct the deviation, we shall see the object in its correct place, but it will be tinted with the prismatic spectrum. It is therefore possible to balsam together a series of flint and crown prisms, and thereby obtain a considerable amount of dispersion, that is length of spectrum, without scarcely any deviation. It will be obvious that a number of prisms absorb a large amount of light, and therefore, to a certain extent, the length of the spectrum of an object is limited by its brightness. From the same cause the spectrum of the Sun can be enlarged to comparatively any extent, and, therefore, only a narrow strip of solar light is admitted by a slit. This strip can be taken from a part of the sun’s surface crossing a spot, and therefore allowing the spectrum of the spot to be seen; or, if from the sun’s limb bringing into view the spectrum of the corona and prominences, and even the prominences themselves. The focal image of a star, being a minute bright point, will illuminate a very small portion of the slit, and will appear in the spectroscope as a coloured line; and therefore a cylindrical lens is used to elongate the spot to a bright line, and give breadth to the spectrum, before the light passes through the slit. It being very difficult to keep the star-line exactly opposite the slit except with a most accurate driving clock; it was soon noticed that the slit was unnecessary, as there was a sufficient line of light formed by the cylindrical lens alone; such a spectroscope can be even used on an alt-azimuth. A spectroscope can therefore be used for stellar purposes, with or without a slit, a remark also applicable to the observation of nebulae, comets, and other objects. Both convex and concave cylindrical lenses have been used.

The method of using all forms of spectroscopes being nearly similar, the following hints on observing different phenomena may prove useful to the possessor of any spectroscope, though, in respect to solar details, they are specially applicable to powerful instruments.

In order to properly observe the solar prominences a powerful spectroscope, applied to an equatorially mounted telescope, driven by clockwork, is necessary. But as they have been glimpsed by a comparatively small spectroscope on an alt-azimuth, the observer should not despair of seeing them with moderate means..

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