in the Manufacture and Theory of Gratings. 471 



to the centre of the mirror, and from its central point, with a 

 radius equal to half the radius of curvature draw, a circle ; this 

 circle thus passes through the centre of curvature of the mirror 

 and touches the mirror at its centre. Now, if the source of 

 light is anywhere in this circle, the image of this source and 

 the 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, somewhat impairs the light, espe- 

 cially of the higher orders; but the introduction of a cylindrical 

 lens greatly obviates this inconvenience. 



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

 the different 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 contrivance 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 photogra- 

 phic 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 coloured on a nearly white ground. 

 Bv micrometre measurement of such superimposed spectra, 

 •2 N 2 



