of determining the Surface- Tension of Liquids. 593 



hanging between the two gives the direction of the vertical.; 

 The apparatus is then photographed in the following 

 manner : — 



A horizontal beam of parallel light is produced by an 

 electric spark placed at the focus of an achromatic lens of 

 long focal length (about 1*5 metres), and is thrown on the 

 apparatus. The shadow cast by the tubes is then photo- 

 graphed on a plate held vertically as near to them as 

 possible. 



The negative is then measured under a sliding microscope. 

 It is coupled, film to film, with a " reseau " plate on which is 

 ruled a set of squares (side = 0*5027 cm.), and placed on an 

 inclined-plane so that it has freedom of up and down move- 

 ment. The microscope, which is capable of motion sideways, 

 has two scales at right angles to each other in its focal plane. 

 The glass plate on which these scales are ruled can be moved 

 in the focal plane by a micrometer-screw. The reseau plate 

 is first adjusted so that the rulings on it are parallel and 

 perpendicular to the shadow of the plumb-line on the plate. 

 By rotating the eyepiece and the photographic plate on the 

 stage, the scales in the focal plane are adjusted parallel 

 to the rules on the reseau plate and to the lines of travel of 

 the microscope and the photographic plate. 



The objective of the microscope (which is a small one) 

 produces very little magnification, but the eyepiece is a 

 fairly powerful one. The scales in the focal plane (1 

 division = jq mm «) serve only as auxiliaries, for the readings 

 made on them can easily be expressed in terms of the side 

 of the squares on the reseau plate, the absolute value of which 

 is known. The x and y coordinates of any point on the 

 meniscus can thus be measured and reduced to centimetres. 

 What was directly read off was not the y coordinate but 2y. 

 From these measurements the radius of curvature at the 

 vertex of the curve can be deduced in the manner mentioned 



above. If the variation of #- is not neoliaible, then x .is 

 written equal to Wt 



aif + by\ 



since the curve is symmetrical about the axis of x. From 

 the observational equations 



x l = ay\* + byf, &c, 



two normal equations for the constants a and b can be 



