On the Spectroscope and its Applications

Abstract

III.SO far, I have spoken of spectroscopes as spectroscopes -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. An instrument with one prism is usually 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. 13; 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. 14 is of an instrument which historically is extremely interesting, as being the one with which Kirchhoff made his most elabo-rate and accurate maps of the solar spectrum; it is furnished with a battery of four large prisms, which give an enormous deviation and dispersion. There is no reason why spectroscopes of many more prisms should npt 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. 15. 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 dis-'persion, 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 moveable arm, which can be directed to any part of the spectrum that may be required; and the outer edge of the circle along which the telescope moves may be graduated with an accurate scale of degrees, which can be divided with more or less minuteness, according'to the'precision in the exact position of the dark lines, &c., in various spectra required. In this method the line to be measured is brought into the centre of the field of view of the observing telescope, and the position of the telescope read off. Of course if the line measured is situated in the red end of the spectrum, the telescope will be in a different position to that it will occupy if the line be in the blue end. The second method of measurement may be gathered from Fig. il. It consists of a short tube carrying at its outer extremity a small photographic scale, which is illuminated by a candle flame; the light passing from the photographic scale is rendered parallel and thrown on the surface of the prism by means of a lens in the tube carrying the scale, and is reflected by the last surface of the prism up the observing telescope, so that it is seen