scopes of many prisms it is essential that there should be some arrangement by which each part of the spectrum should be observed with each prism at the angle of minimum deviation for that ray. This may be done in many ways, and the beam may be made to pass back again through the prisms, thus doubling the amount of dispersion. On these points I shall have more to say presently. Another important consideration, besides the purity of the

material, is the perfect figure of the slit. You might imagine that the slit of a spectroscope was perfectly easy to make; but, judging by the results of the manufacture, it is extremely difficult, for a perfect slit is still very rare, the best being made by Steinheil of Munich. Mr. Browning has suggested making the slit of a compound of gold, which will not rust, or be acted upon much by temperature,

and which also will take a good figure without any

very great difficulty.

So far, I have spoken of spectroscopes as spectro

FIG. 15.-Steinheil's form of four-prism spectroscope: arrangement of slit shown separately.

scopes-as one of the instruments the improvement of which should be cared for by every student in science. Their applications will come after. As

may be imagined, spectroscopes are now constructed with one, two, three, four, or more prisms, the number depending on the purpose for which they are to

be employed. The instrument shown in Fig. 13 may be called a chemical spectroscope, for an instrument of this kind is now almost as important and essential

in a chemical laboratory as a balance. Spectroscopes are also constructed with two prisms, as shown in Fig. 14; these are used in cases when rather more dispersion is desired than can be obtained with the one-prism instrument. When, however, any accurate and elaborate work has to be done, such as in carrying out original investigations, more prisms have to be employed. The engraving given in Fig. 15 is of an instrument which historically is extremely interesting, as being the one with which Kirchhoff made his most elaborate and accurate maps of the solar spectrum; it is furnished with a battery of four large prisms, which give a great deviation and dispersion. There is no reason why spectroscopes of many more prisms should not be employed, except that they require to be worked only with strong lights, as light is here so much dispersed or spread out that a feeble spectrum would be almost lost.

As the principle of construction is almost the same in all kinds of spectroscopes, we had better commence by a description of the simplest form, namely that with one prism, as shown in Fig. 16. It will be seen to consist of a circular table, supported by a pillar and three legs, carrying three lateral tubes; the right-hand tube is called the collimator, and holds at its outer extremity the fine slit, the width of which can be regulated to a nicety by a micrometer screw; the other end of the collimator is furnished with a lens, which serves to collect the rays of light coming from the slit, and to render them parallel before falling on the prism in the centre of the table. The prism is so placed and fixed by a

clamp that the light entering the slit from the source of light, shown in the figure as a gas lamp, strikes it and leaves it at what is called the angle of minimum deviation, a term which has already been explained; after passing through the prism, in which the light. undergoes both deviation and dispersion, the spectrum

is observed by the telescope on the left, which is simply a small astronomical telescope of low magnifying power. There are two methods of measuring spectra. The telescope may be attached to a movable arm, which can be directed to any part of the spectrum that may be required; and the outer edge