1889.] Lt.-Col. J. Waterhouse — On BoivlancVs Diffraction Gratings. 7 



with, rackwork by which the slit can be drawn in and out so that its 

 plane may be brought exactly over a pivot fixed on the sliding bar 

 carrying the Ys, and to which pivot is also attached a second bar the other 

 end of which is fixed by a pivot placed below the camera at a central 

 point between the grating and the photographic plate. The apparatus 

 is adjusted so that the distances from the central pivot of the camera to 

 the grating, the photographic plate and the slit are all equal to half the 

 radius of curvature of the grating, and the lines of the s|3ectrum should 

 then appear in sharp focus when viewed through the eye-piece placed so 

 as to focus the centre of the ground-glass of the camera. As the camera 

 is drawn away from the slit the violet of the first order comes into 

 view, followed by the indigo, blue, &c. Beyond the red end of the 

 spectrum of the first order there is an apparently dark space, which, as I 

 remarked above, is really filled with lines of the invisible ultra-violet of 

 the second order. About b of the second order, the violet lines of the 

 third order make their appearance, and below the D lines of the second 

 order come the H lines of the third order ; so that if it is required to 

 photograph these D lines a yellow screen must be used ; otherwise the 

 photograph obtained will show the H and other violet lines most pro- 

 minently, as one of the plates I have with me will show. 



With the apparatus as it stands, the D lines of the third spectrum 

 are just beyond the travel of the camera ; but by a little adjustment of 

 the tube they could be seen. The b lines of this spectrum can readily 

 be seen, with very considerable dispersion. 



In working the apparatus, it is best to set the plank carrying the 

 tube in the direction of the heliostat, and then fasten it down by clamps 

 or screws on to a table large enough to carry the whole apparatus and 

 admit of the camera being opened out to its widest extent. As the 

 end of the camera moves round on a circular arc, it is quite easy to mark 

 off on the table the positions of the principal lines on a corresponding 

 arc, so that the centre of the camera can be brought to any particular 

 line in either of the orders, at once. If greater accuracy is required, 

 scales of wave-lengths can be marked off, in the same way, for the dif- 

 ferent orders, either along the arc or along the grooved plank in which 

 the tube slides. 



I am sorry I am unable to show you the powers of the instrument, 

 but even with an electric-light apparatus it would be difficult to do so 

 satisfactorily. The capabilities of the new gratings can only be fully 

 appreciated by observing the solar spectrum ; and without exaggeration 

 I may say that it is one of the most fascinating things I know to watch 

 the lines in different parts of the spectrum under their varying aspects 

 and groupings, and to feel that in them may be the key to mighty and 

 yet unfathomed secrets of the universe. 



