436 Prof. R. W. Wood on the Selective Dispersion of 



of the instrument by means of a quartz lens. An image of 

 the fringe system was now projected upon the slit of a quartz 

 spectrograph by means of a quartz fluorite achromatic lens. 

 This gives us a spectrum of the arc traversed by horizontal 

 black lines (the interference fringes), and if we have perfect 

 compensation of the optical paths, and are at the centre of 

 the system, the fringes will run in a horizontal direction 

 throughout the entire spectrum. As a matter of fact it was 

 found that no fringes at all w r ere visible in the ultra-violet 

 region, and that they sloped in a very oblique direction in 

 the violet, the slope increasing with a decrease of wave-length. 

 This circumstance is due to the use of the two quartz end 

 plates as a compensator for the fluorite plate : the dispersion 

 of the two substances is quite different, and the thickness 

 traversed is not the same. The movable mirror of the 

 instrument was now moved back a little and a second photo- 

 graph taken. Matters were now worse than before, so the 

 mirror was advanced a little ahead of the position which it 

 occupied (perhaps 200 wave-lengths) when the coloured 

 fringes were visible with white light. The fringes could 

 now be followed through the entire spectrum, though they 

 were very oblique at the extreme ultra-violet end. A further 

 advance of the mirror brought them approximately horizontal 

 in the region of the 2536 line of mercury, where all of the 

 work in the present case was to be done. Viewed with an 

 eyepiece the spectrum now showed nearly vertical fringes in the 

 red and orange, and very oblique ones in the yellow and green. 

 A rather poor photograph of the entire spectrum is repro- 

 duced on PL III. fig. 9, which, however, gives a fair idea of the 

 change in the slope of the fringes with change of spectral range. 

 At the right-hand end of the picture (blue) the fringes slope 

 down to the right at an angle of about 45°. Tn the middle 

 (upper ultra-violet) they are horizontal, while at the left 

 (lower ultra-violet) they slope down to the left. This method 

 of studying dispersion was first used by Puccianti in his 

 study of the vapours in metallic arcs. It has the advantage 

 of giving us a photographic record of the retardations or 

 accelerations, as we may for convenience term them, of 

 waves of various lengths in the vicinity of absorption bands. 

 In the present case we are dealing with a single absorption 

 band only, the 2536 line of mercury, which fortunately falls 

 in the middle of a group of strong iron lines. When mercury 

 vapour is formed in the quartz tube, the fringe systems in 

 the various iron lines are shifted by different amounts, and in 

 different directions. A photograph of the iron spectrum in 

 coincidence with that of the quartz mercury arc, in the 



