1867.] Colouring-matters hy the Spectrum Microscope. 435 



made up of alternate black and coloured bands, evenly distributed over the 

 v/bole spectrum. The number of these depends on the thickness of the 

 depolarizing plate, so that we may have, if we please, almost innumerable 

 tine black lines, or fewer, broader bands, black in the centre and shaded off 

 at each side. These facts are of course easily explained by the interference 

 of waves. It would, I think, be impossible to have a more convenient or 

 suitable scale for measuring the spectra of coloured solids and liquids. If 

 we use a micrometer in the eyepiece, an alteration in the width of the slit 

 modifies the readings, and the least movement of the apparatus may lead 

 to error, whereas this scale is not open to either objection. Besides this, 

 the unequal dispersion of the spectrum makes the blue end too broad, so 

 that a given width, as measured with a micrometer in the eyepiece, is not 

 of the same optical value as the same width in the red. The divisions in 

 the interference-spectrum bear, on the contrary, the same relation to the 

 length of the waves of light in all parts of the spectrum, and no want of 

 adjustment in the instrument alters their position. As will be seen from 

 the drawing (fig. 1), the unequal dispersion makes the distance betweeen 

 the bands in the blue about twice as great as in the red. The perfection 

 of a spectrum would be one in which they were all at equal intervals ; but 

 possibly no such uniform dispersion could be produced. By having a 

 direct-vision prism, composed of one of flint-glass of 60°, and two of 

 crown-glass of suitable angle, we can place it over the eyepiece, and may 

 diminish the dispersion at the blue end, or increase that at the red 

 end, by turning it in one position or the other, and thus see either 

 end to the greatest advantage. It is, of course, very easy to draw 

 spectra on this principle, and give all parts equal prominence, and not an 

 unduly compressed red, and an unduly expanded blue end. Thus drawn, 

 the spectra are far more uniform in many of their characters, and some 

 genei-al laws are at once apparent that otherwise might have been entirely 

 overlooked ; and on this account I shall adopt this system in those figured 

 in this paper. It is, in fact, merely representing the actual measurements 

 by drawings, without being at the trouble of distorting them, so as to be 

 like naturally distorted spectra. 



Since the number of divisions depends on the thickness of the interfer- 

 ence-plate, it became necessary to decide what number should be adopted. 

 At first 1 thought that ten would be most suitable ; but, on trying, it 

 appeared to me too few for practical work. Twenty is too many, since it 

 then becomes extremely difiicult to count them. Twelve is as many as 

 can be easily counted ; it is a number easily remembered, gives sufificient 

 accuracy, and has a variety of other advantages. With twelve divisions 

 the sodium-line d comes very accurately at 3J-; and thus, by adjusting 

 the plate so that a bright sodium-hne is hid in the centre of the band, 

 when the Nicol's prisms are crossed, it is accurately at 3|, when they are 

 arranged parallel, so as to give a wider field. The general character of the 

 scale will be best understood from the following figure, in which I have 



2 o 2 



