1889.] Lt.-Col. J. Waterhouse — On Rowland's Diffraction Gratings. 5 



The plane gratings, as above remarked, are used much in the 

 same way as prisms, either for photography or observing with a teles- 

 cope. Light is passed through a fine slit attached to a collimating 

 tube and lens, so that parallel rays from the slit fall on the centre of 

 the grating and accurately parallel to the ruling. The image of the 

 slit is then viewed through a telescope or received on the focussing 

 glass of a photographic camera, mounted so that it can revolve on the 

 same axis as the grating. Then by inclining the grating, or by re- 

 volving the telescope or camera, the spectra of the different orders are 

 brought into view. When working with the higher orders of spectra 

 special means have to be adopted for separating the overlapping spectra, 

 either by a prism placed between the grating and the telescope or camera, 

 or by using suitably coloured screens in front of the slit. Thus in 

 photographing the red end of the spectrum, a red glass or other screen 

 is always required to cut off the ultra-violet rays. 



With plane gratings the focus changes in different parts of the 

 spectra according to the refrangibility of the rays, and the foci of 

 rays of different orders overlapping do not correspond. This latter 

 property is often of use in measuring wave-lengths and coincidences of 

 lines, in the various spectra. 



The concave gratings were first introduced some six years a^o by 

 Professor Rowland, of the Johns Hopkins University, Baltimore, who 

 made a greatly improved machine for ruling and adapted it to rule 

 concave surfaces of speculum metal. 



In these gratings, the lines of the spectrum, formed by the image 

 of a fine slit, come to a focus at a distance from the grating equal to 

 its radius of curvature, and, therefore, if the slit, the ruled surface of 

 the grating, and the eyepiece of the telescope or photographic plate 

 are always situated on the circumference of a circle of which the radii 

 are equal to half the radius of curvature of the grating, the spectrum 

 will always be in focus at any point on the circumference of the circle. 

 Moreover, when the centre of the grating and the centre of the eye-piece, 

 or photographic plate are in the same straight line, the line of foci will 

 be perpendicular to the direction of the light, and the spectrum will be 

 normal, whatever the position of the slit, provided it is on the circle. 



It will be seen, therefore, that these gratings are especially suitable 

 for photographic work — though from the amount of dispersion given, even 

 by comparatively small gratings, the extent of spectrum included on a 

 plate at any one time is comparatively small, unless long plates be used 

 and curved to correspond with the curvature of the circle of which the 

 focal length of the grating is the radius. 



The camera in which the concave grating exhibited this evening is 



