GRATINGS FOR OPTICAL PURPOSES 489 



different orders of the spectra are all brought to focus on this circle. 

 The word focus is hardly applicable to the case, however, for if the 

 source of light is a point the light is not brought to a single point on 

 the circle but is drawn out into a straight line with its length parallel 

 to the axis of the circle. As the object is to see lines in the spectrum 

 only, this fact is of little consequence provided the slit which is the 

 source of light is parallel to the axis of the circle. Indeed it adds to 

 the beauty of the spectra, as the horizontal lines due to dust in the slit 

 are never present, as the dust has a different focal length from the lines 

 of the spectrum. This action of the concave grating, however, some- 

 what impairs the light, especially of the higher orders, but the intro- 

 duction of a cylindrical lens greatly obviates this inconvenience. 



The beautiful simplicity of the fact that the line of foci of the dif- 

 ferent orders of the spectra are on the circle described above leads 

 immediately to a mechanical contrivance by which we can move from 

 one spectrum to the next and yet have the apparatus always in focus; 

 for we only have to attach the slit, the eye-piece and the grating to three 

 arms of equal length, which are pivoted together at their other ends 

 and the conditions are satisfied. However we move the three arms the 

 spectra are always in focus. The most interesting case of this contriv- 

 ance is when the bars carrying the eye-piece and grating are attached 

 end to end, thus forming a diameter of the circle with the eye-piece at 

 the centre of curvature of the mirror, and the rod carrying the slit 

 alone movable. In this case the spectrum as viewed by the eye-piece 

 is normal, and when a micrometer is used the value of a division of its 

 head in wave-lengths does not depend on the position of the slit, but 

 is simply proportional to the order of the spectrum, so that it need be 

 determined once only. Furthermore, if the eye-piece is replaced by a 

 photographic camera the photographic spectrum is a normal one. The 

 mechanical means of keeping the focus* is especially important when 

 investigating the ultra-violet and ultra-red portions of the solar 

 spectrum. 



Another important property of the concave grating is that all the 

 superimposed spectra are in exactly the same focus. When viewing 

 such superimposed spectra it is a most beautiful sight to see the lines 

 appear colored on a nearly white ground. By micrometric measurement 

 of such superimposed spectra we have a most beautiful method of 

 determining the relative wave lengths of the different portions of the 

 spectrum, which far exceeds in accuracy any other method yet devised. 

 In working in the ultra-violet or ultra-red portions of the spectrum we 



