in Theory and Practice. 37- 



wliile for photographing in the ultra-violet it is best to have 

 a 20,000 grating, wiili a ruled space of 5^ in. on a 6 in. 

 polished surface. The radius of curvature is generally 21'5 ft. 

 The photographic plates are 19 in. long, 2 in. wide, and -]^4 in. 

 thick. This thickness allows the plates to be bent to the 

 required radius without breaking. They are flowed with an 

 ammonia emulsion by Professor Rowland himself, and register 

 from 15-20 on Warnerke's sensitometer. Quick plates give 

 too coarse an eflPect for enlargement. For short focus gratings, 

 such as may be used for gaseous spectra or direct stellar 

 spectra, Professor Rowland has suggested the use of paper 

 or celluloid negatives, as they can be bent to a small radius. 



The micrometer-eyepiece used is more like a dividing- 

 engine than an ordinary micrometer. It has a run of 

 5 inches, and the screw is to all purposes perfect, having been 

 made according to the directions given by Professor Rowland 

 in his article on the Screw in the Encyc. Brit. vol. xxi. 

 p. 552. 



Hoods of black cloth to keep out stray light are necessary 

 at the slit, and at the camera-box, where one should extend 

 halfway to the grating, as even the darkest room has some 

 light in it. 



Adjuatments. 



The adjustment of these various parts of the apparatus is 

 comparatively simple. The two beams carrying the grating 

 and camera-box are made as level as possible and placed at 

 right angles by the " 3, 4, 5 " rule. The two axes at the ends 

 of the girder must be made parallel, lohile the girder is under 

 stress. To do this the girder is supported at its ends on 

 two "horses;" and the axes are adjusted by the control- 

 screws until the two are vertical. This is the most difficult 

 adjustment. 



The camera-box, grating- holder, and slit are put in place 

 at the proper height. Most gratings give a brighter spectrum 

 on one side than on the other ; and so, before placing the 

 grating on its holder, it must be examined to see which side 

 should be used. A candle is held at the centre of the camera- 

 box, which is kept directly over the axis of the carriage ; 

 and the grating is turned and the girder lengthened until the 

 flame and its image coincide. By this, the grating is placed 

 perpendicular to the girder, and the girder itself is given the 

 correct length. The camera-box is then made vertical by a 

 plumb-line. To adjust it perpendicular to the girder, a piece 

 of plate-glass is fastened to its face, and a candle is held on 

 the girder near the grating. The camera-box is then revolved 



