_Mapping the Spectra._--In the sectional view given of the micro-spectroscope (Fig. 196), the internal construction of the instrument is shown, and the arrangement made for throwing a bright point on to the surface of the upper prism is clearly seen. The mapping out is accomplished by means of a photographic scale fixed as a standard spectrum (Fig. 198), in the position of A A, illuminated by the small mirror at R, and focussed by a small lens at C, so that on looking into the instrument one can see the spectrum accurately divided into one hundred equal parts, and scale readings can be made at once; the only precaution needed is to be sure the D (or the sodium line, if D cannot be got) always stands at the same number on the scale. To map absorption spectra on this scale we have to lay down a line, as many millimetres long as there are divisions in the scale, and mark the position of the bands on this line. Mr. Browning supplies scales printed off ready for use. But the mapping out of spectra, as Mr. Sorby pointed out, requires some consideration; since the number of divisions depends on the thickness of the interference-plate, it becomes necessary to decide what number should be adopted. Ten it was thought would be most suitable; but, on trial, it appeared to be too few for practical work. Twenty is too many, since it then becomes extremely difficult to count them. Twelve is as many as can well be counted; it is a number easily remembered, is sufficiently accurate, and has other practical advantages. With twelve divisions the sodium-line 0 comes very accurately at 3-1/2; thus, by adjusting the plate so that a bright sodium-light is brought into the centre of the band, when the Nicol’s prisms are also crossed accurately at 3-1/2, parallelism is secured, together with a wider field of observation. The general character of the scale will be best understood from the following figure, in which the bands are numbered, and given below the principal Fräunhofer lines. The centre of the bands is black, and they are shaded off gradually at each side, so that the shaded part is about equal to the intermediate bright spaces. Taking, then, the centres of the black bands as 1, 2, 3, &c., the centres of the spaces are 1-1/2, 2-1/2, 3-1/2, &c., the lower edges of each 3/4, 1-3/4, &c., and the upper 1-1/4, 2-1/4, &c., we can easily divide these quarters into eighths by the eye: and this is as near as is required in the subject before us, and corresponds as nearly as possible to 1/100th part of the whole spectrum, visible under ordinary circumstances by gaslight and daylight. Absorption-bands at the red end are best seen by lamp-light, and those at the blue end by daylight. [Illustration: 0 1 2 3 4 5 6 7 8 9 10 11 12 (Red end.) (Bue end.) A B C D E _b_ F G Fig. 200.-- ] On this scale the position of some of the principal lines of the solar spectrum is about as follows:--