52 Lord Rayleigh on the Crispations of Fluid 
of the coexistence of the two sets of ridges and furrows in p3r- 
pendicular directions (x,y). In the adjoining figure tl e thick 
lines represent the ridges, and the thin lines the furrows, of 
the two systems at a moment of maximum excursion. One 
quarter period later the surface is flat, and one half a period 
later the ridges and furrows are interchanged. The places of 
maximum elevation and depression are the intersections of the 
thick lines and of the thin lines, not distinguishable bv ordi- 
nary vision ; and these regions will appear like holes in the 
sheet of colour. The nodal lines, where the normal depth of 
colour is preserved, are shown dotted ; they are inclined at 
45°, and pass through the intersections of the thick lines with 
the thin lines. The pattern is recurrent in the directions of 
both x and y, and in each case with an interval equal to the 
real wave-length (X). The distance between the bright spots 
measured parallel to x or y is thus \ ; but the shortest dis- 
tance between these spots is in directions inclined at 45°, and 
is equal to i </ 2 . X. 
In order to determine the relation of the frequencv of the 
liquid vibrations to that of the bar, an apparatus was fitted up 
capable of giving an intermittent view of the vibrating system. 
This consisted of a blackened paper disk pierced with three 
sets of holes, mounted upon an axle, and maintained in rota- 
tion by a small electromagnetic engine of Apps's construction. 
The whole was fastened to one base- board, and could be moved 
about freely, the leading wires from the batterv being flexible. 
