( 221 ) 



point of the ociilai' wires, and the angle of inclination of the curve 

 at this point can be measured. The method which also deserves 

 recommendation for the measurement of other, especially capillary- 

 electrometric curves, leaves little to be desired as to ease and 

 quickness. Its accuracy can be judged from the following results. 



In the reticular scales, which together with the movements of the 

 quartz thread are photographed on the plates, the ordinates are not 

 perfectly perpendicular to the abscissae, which must be ascribed to 

 the circumstance that the image of the slit, formed by the cylindrical 

 lens on the photographic plate, is not perfectly perpendicular to the 

 direction of motion of the sliding frame. I did not carry out my 

 original intention of correcting this error by a calculation, since 

 the deviation only amounts to 0°.3. 



Besides we can, when measuring angles of inclination less than 

 45°, take an absciss as base, when above 45° an ordinate, by which 

 proceeding the deviation becomes of still less importance. 



But from measurements of this deviation by means of the turning 

 eye-piece, it appears that the measurements of angles can be made 

 with errors not exceeding 0°.l. This will be seen from the following 

 table. 



TABLE I. 



No nearer explanation is wanted. In the last column but one we 

 find the results of the measurement of the angles under which in 

 various photographic plates the abscissae aie cut by the ordinates. 

 Taking for the average value an angle of 89°, 7, we see that the 

 greatest deviation from this value is only 0,°1. 



It must be remarked, however, that the direction of the tangent 

 in a point of the curve cannot be determined so accurately as the 



