102 ScientifiG Proceedings, Royal Dublin Society. 



the revolving prism-table, U the clampiug screw for fastening the 

 table when the angle of minimum deviation is obtained for the 

 cadmium line 17. V is a cap to the collimator tube for exclud- 

 ing rays of strong light. Over this cap the prism-box is fitted 

 having an oval opening at one side to admit of a sufficient amount 

 of movement of the collimator tube to the right or left to secure 

 proper adjustment. There is a round aperture opposite the 

 lens for the passage of the refracted rays. 



Care should be taken to secure prisms cut perfectly true at the 

 base in addition to having good faces. After fixing the prism in 

 the desired position, it is necessary to adjust the collimator tube 

 very carefully to its proper angle, otherwise the best results will 

 .never be attained when photographing metallic lines. The colli- 

 mator tube being three feet in length is placed on a separate 

 stand. The slit end terminates in a small mahogany box with a 

 hinged lid. At the extremity of the box the slit is fixed. This 

 box serves two purposes ; first, when the absorption spectra of 

 volatile liquids are being examined the substance is placed in a 

 little cell of quartz which stands in the box behind the slit, thus 

 lessening the loss of rays. When the lid of the box is shut the 

 liquid if excessively volatile is not inflamed by the spark, and as 

 the vapour may fill the collimator tube, its absorptive power is 

 readily examined notwithstanding its rapid evaporation. The 

 second purpose of this box is to enable the experimenter to judge 

 in Avhat direction to move the electrodes so that the spark may 

 exactly fall upon the slit and pass in a straight line down thp 

 collimator tube. This is done by means of a piece of card which 

 fits just inside the box and has a vertical line ruled down its 

 centre, when the spark is exactly opposite to the slit, the ray of 

 lio-ht strikes the centre of the card. A tube at the far end of the 

 collimator near the prism box about one-quarter inch in diameter 

 and three inches long, projects upwards at right angles and serves 

 for the introduction of gases into the tube, which may require 

 examination. It is also necessary when examining volatile 

 liquids to exhaust the tube of the vapours they evolve by dravv- 

 ino- a current of air through the tube, otherwise subsequent ex- 

 periments might be vitiated by the residue of a highly absorbent 

 vapour. 



The cells for holding solutions are cut from pieces, of glass tube 



