The actual construction of the astronomical spectroscope varies greatly according to its use. In observations on the sun the distant slit is brought nearer for convenience by placing it in the focus of a small objective pointed toward the prisms (the collimator) and the spectrum is viewed by a telescope of moderate magnifying power to disclose more of detail. Also, since there is extremely bright light available, very great dispersion can be used, obtained by several or many prisms, so that the spectrum is both fairly wide, (the length of the slit) and extremely long. In trying to get the spectrum of a star the source is a point, equivalent to an extremely minute length of a very narrow slit. Therefore no actual slit is necessary and the chief trouble is to get the spectrum wide enough and bright enough to examine. The simplest form of stellar spectroscope and the one in most common use with small telescopes is the ocular spectroscope arranged much like Fig. 140. This fits into the eye tube of a telescope and the McClean form made by Browning of London consists of an ordinary casing with screw collar _B_, a cylindrical lens _C_, a direct vision prism _c_, _f_, _c_, and an eye-cap _A_. The draw tube is focussed on the star image as with any other ocular, and the light is delivered through _C_ to the prism face nearly parallel, and thence goes to the eye, after dispersion by the prism. This consists of a central prism, _f_, of large angle, made of extremely dense flint, to which are cemented a pair of prisms of light crown _c_, _c_, with their bases turned away from that of _f_. We have already seen that the dispersions of glasses vary very much more than their refractions so that with proper choice of materials and angles the refraction of _f_ is entirely compensated for some chosen part of the spectrum, while its dispersion quite overpowers that of the crown prisms and gives a fairly long available spectrum. The cylindrical lens _C_ merely serves to stretch out the tiny round star image into a short line thereby giving the resulting spectrum width enough to examine comfortably. The weak cylindrical lens is sometimes slipped over the eye end of the prisms to give the needed width of spectrum instead of putting it ahead of the prisms. A small instrument of this kind used with a telescope of 3 inches to 5 inches aperture gives a fairly good view of the spectra of starts above second or third magnitude, the qualities of tolerably bright comets and nebulæ and so forth. The visibility of stellar spectra varies greatly according to their type, those with heavy broad bands being easy to observe, while for the same stellar magnitude spectra with many fine lines may be quite beyond examination. Nevertheless a little ocular spectroscope enables one to see many things well worth the trouble of observing. [Illustration: FIG. 140.—McClean Ocular Spectroscope.]