294 Dr. J. H. Vincent on 



falls off very rapidly as the distance from a source increases 

 owing to viscosity. 



Fig. 7. Frequency 190. 



This is the exact analogue of Young's interference experi- 

 ment. In the dark region a point-source is just visible. 

 The style is seen as a black line. The darkness is due to 

 ripples of considerable amplitude being employed. The 

 ripples proceeding from this source meet a bridge made of 

 microscope cover-glass ; the bridge has two arches and one 

 pillar. The width of the pillar is 4- 5 rnilliin., and of the 

 arches on each side '75 millim. The whole was floating, 

 and the source was on a line bisecting the bridge at right- 

 angles. This line is the median line of the central band of 

 maximum motion. 



The bridge moves as a whole, its vibration being forced by 

 the ripples from the source ; this effect was made small by 

 having the approaches to the bridge very long. The region 

 on the same side of the bridge as the source is marked by 

 Lloyd's bands referred to in former papers. 



Fig. 8. Frequency 190. 



The two point-sources are moving in the same phase. The 

 surface near the fork is marked by the usual interference- 

 bands, while the bands on the other side of the sources are 

 displaced by the influence of a drop of water floating on the 

 mercury. The exact meaning of the white marks in the 

 middle of the drop is uncertain. 



Being unwilling to disfigure the original negative a con- 

 tact positive on glass was made. On this the diagram was 

 drawn and a new negative made by contact. From this 

 negative, fig. 8 is printed. This method eliminates error 

 which would arise from measuring a print. 



The line AB is drawn through the point-sources and is 

 bisected at right angles by the line upon which the point P is 

 marked. On the side remote from the drop this line is in the 

 middle of the central band of maximum motion. The band upon 

 which P is situated is moved nearly half a fringe to the left. 



This illustrates the crucial test of Newton's emission theory 

 devised by Arago. 



Fig. 9. Frequency 170. 



When the slit which emits the light to be diffracted by a 

 Eowland grating is placed at the centre of curvature of the 

 latter, the light reaches the grating in the same phase. The 

 spaces between the rulings may then be regarded as a serk s 

 of line-sources if we look at the matter from a two-dimensional 

 standpoint. 



