328 Prof. G. Quincke on the Edge-angle and 



of 110 millims. length and 2 millims. diameter, is fastened 

 with cork a mirror S (of silvered glass) of 30 millims. height 

 and 15 millims. breadth, and upon the other side a vertical 

 divided circle K, of 45 millims. diameter, printed on card and 

 divided to whole degrees. An arm B C of the same steel wire 

 serves to rotate the divided circle and mirror in a hole C D 

 bored through a large cork which has been forced tightly onto 

 a vertical glass rod Gr, of 250 millims. length and 8 millims. 

 diameter. Two diametral arms of brass, M M 1; which are like- 

 wise fastened into the large cork, allow the rotation to be read 

 offto0°-l. 



That the reflecting surface S stands parallel to the axis of 

 rotation A A ± is verified, as in the ordinary goniometer, by 

 rotating through 180°. 



The mirror S must be placed horizontal and near to a larger, 

 straight-edged, horizontal mirror G 2 , so that the images of a 

 horizontal window-bar parallel to A A 1 reflected in each mirror 

 may coincide. Upon the horizontal mirror Gr 2 , and as near 

 as possible to the mirror S, is laid the fixed plate with the flat 

 drops whose edge-angle is to be determined. The eye is 

 lowered until the reflected image of the sky-lit window has 

 just disappeared at the curved surface of the drop ; and the 

 mirror S is turned about the axis A A x until the upper edge 

 of the bright image of the window appears on a level with the 

 edge of the drop. Then the mirror stands parallel to the last 

 element of the free surface of the drop, and the rotation from 

 the first position to the second, measured on the divided circle, 

 gives directly the acute edge-angle 6 with a precision sufficient 

 for the purpose in question. 



4. Influence of Height of Fall and of Impurities upon the 

 Edge-angle. 



The edge-angle 6 is found to be the smaller as the height h 

 from which the drops fall upon the plate is greater. 



For water, and a glass plate carefully cleansed with alcohol, 

 water, and a clean linen cloth, I found : — 



7i = millim. 

 0=22° 34' 



20 millims. 

 12° 44' 



130 millims. 

 7° 13' 



With another plate of the same glass plate better cleansed : — 



h=0 millim. 10 millims. 



0=12° 29' 9° 8' 



100 millims. 

 5°54 / 



And two minutes later, repeating the experiment on another 

 spot of the same plate : — 



0=16° 49' I 14° I 8° 41' 



