﻿Measurements of Pendent Drops. 421 



the magnification is known, since the diameter of the tube 

 on the photograph can be measured and compared with the 

 actual diameter of the tube used. 



After a few preliminary trials, a microscope objective was 

 substituted for the camera lens, giving a much higher mag- 

 nification. The drop itself now occupied almost the whole of 

 the photographic plate, thus rendering two photographs 

 necessary, one of the drop alone, another with a lower 

 magnification showing both the drop and the horizontal 

 surface of the fluid in the beaker. 



Ajter focussing the edge of the drop sharply on the screen, 

 and before taking the photograph, a little fluid was run out 

 of the siphon-tube by screwing up the adjustable table 

 carrying the beaker. This ensured the cleanliness of the 

 surface of the drop. The photograph under high magnifica- 

 tion was then taken, the microscope objective removed and 

 camera-lens substituted, the drop being now refocussed and 

 the second photograph taken. The whole interval between 

 exposing the surface of the drop to the air and taking the 

 final photograph need not be much more than a minute, and 

 the surface may fairly be assumed to be uncontaminated 

 during this interval. The photographs so taken were 

 measured by means of a travelling microscope provided with 

 verniers reading to '001 centimetre, and capable of motion 

 parallel to \he a'xes of a* and y ; a third motion, parallel to the 

 £-axis, providing for the focussing of the microscope. Thus 

 the coordinates of points on the meridional surface could be 

 plotted out, and the coordinates of the vertex read off from 

 the graph so obtained. 



It is evident that if the vertex of the drop be taken as 

 origin, the y-axis being drawn vertically upwards, the con- 

 stant term in the equation to the curve will vanish, and, on 

 account of the symmetry about the axis of y, only even 

 powers of x will appear in the equation. The form of the 

 equation Avill therefore be 



y = bx 2 + cx* + .... 



In the tables given below, two types of equation were 

 tried, — the parabolic form y = ba? } and the biquadratic 

 y = fo»* + car*. 



The exact equation to the surface of the drop is 



where A and B arc constants, h being the distance I rem the 



