6 Lt.-Col. J. Waterhouse — On Rowland's Diffraction Gratings. [Jitf. 



used lias been made up on the principle of the arrangement described by 

 Captain Abney, R. E., in Phil. Trans. Roy. 8oa., Vol. CLXXVII, Part II, 

 1886, but modified so that it works automatically on a pivot placed below 

 the centre of the grating. It consists of a long box or camera fitted 

 at one end with a suitable holder in which the grating is placed so that 

 it may be vertical and its axis centrical with the axis of the collimator, 

 on one side, and to the centre of the photographic plate or eyejuece 

 on the other. This holder has screw adjustments for slightly moving 

 the grating in vertical and horizontal arcs. 



To the other end of the box an ordinary photographic bellows- 

 camera is adapted. The back of this camera is arranged so that the 

 ordinary glass focussing-screen can be removed and its place supplied by 

 eye-pieces which serve for focussing the lines sharply for photography or 

 for observing by eye. The eye-pieces fit in a tube arranged to travel in 

 slides vertically and horizontally, so that it can be brought to any part 

 of the spectrum in any position within the size of the photographic 

 plate. This tube is pivoted so as to be able to view the rays obliquely, 

 which is very important in observing extreme rays, especially with the 

 prismatic spectrum. 



Below the grating end of the camera, and vertically underneath the 

 centre of the grating, is a pivot by which the plank supporting the slit 

 and tube is attached to the camera, This plank has grooves at the sides 

 in which travels a bar carrying the Ys which support the tube. One of 

 these Ts, nearest the slit, is fixed, and the other is moveable, sliding 

 backwards or forwards, according to the length of the tube, and also 

 right and left, so that the circle of light formed by the slit in passing- 

 through the tube may be centered on the grafciug. The Ys are elevated or 

 lowered by means of screws, so that the axis of the tube may be levelled 

 in a line with the centre of the grating and axis of the camera. A rack- 

 work attached to the foremost Y, and worked by means of a Hook's 

 joint attached to a long handle, enables the observer at the camera or 

 eyepiece to gently turn the slit round so as to bring it exactly parallel 

 to the lines of the grating. This adjustment, as Caj)tain Abney has 

 pointed out, is very important in securing good definition. 



The sliding tube is a telescope tube fitted with additional screw 

 joints so that it may be extended or shortened, as required, from about 

 1 foot to 5 feet. 



The outer end of the tube carries the slit, which is adjustable by a 

 micrometer screw. With the concave grating no collimating lens is 

 necessary, as with the plane ones ; and this is a great advantage iu dis- 

 pensing with the use of glass, which always cuts off moi'e or less of the 

 extreme rays at both ends of the spectrum. The end of the tube is fitted 



