220 Messrs. Searle, Aldis, and Dobson on Revolving Table 



a telescope with cross-wires may be used i£ an extra con- 

 verging lens be fitted in front of the objective so as to allow 

 the instrument to be used at short distances. 



The carriage is now moved along the scale into a second 

 position in which the axis of the table is a tangent line 

 to the spherical surface. If, now, the table be turned about 

 its axis, a grain of lycopodium placed at the point of contact 

 of the vertical tangent line and the surface will remain 

 stationary. By using a microscope for observing the 

 lycopodium, this setting can be made with great accuracy. 

 The table may be turned backwards and forwards through 

 an angle of a few degrees about a mean position in which 

 the perpendicular from the centre of curvature of the surface 

 upon the axis of the table approximately coincides with the 

 axis of the microscope. 



The radius of curvature of the surface is now given at once 

 by the difference of the two scale-readings of the carriage in 

 the first and second positions. 



§ 3. When the scale is not in perfect adjustment on the 

 table, the straight line described by the centre of curvature 

 of the spherical surface, as the carriage slides along the scale, 

 will not intersect the axis of revolution of the table. If, 

 however, the shortest distance between these two straight 

 lines be small, and if in both the first and second settings 

 the axis of the microscope point nearly to the centre of 

 curvature of the surface, the error in each of the two settings 

 of the carriage will be very small, provided that, in each 

 case, the table is turned about a mean position in which the 

 scale is parallel to the axis of the microscope. Thus a 

 rough adjustment of the scale on the table is sufficient for 

 any but the most precise work. 



§ 4. The adjustments of the surface to be tested are greatly 

 facilitated by the use of a small lai he-head to form the 

 " carriage " mentioned in the preceding paragraphs ; and 

 we shall describe the method of making the measurements 

 when the latht-head is used. The apparatus is shown in 

 figs. 1 and 2. 



The revolving table rests upon a tripod stand, and the 

 table-top turns about a vertical rod carried by the tripod. 

 The top of the table carries a scale graduated in milli- 

 metres, which can be clamped in any position on the table 

 by the screw seen in fig. 1. Revolving tables of this 

 description are now employed in many laboratories in the 

 determination of the focal lengths of lens-systems by the 

 nodal -point method. The lathe-head and its adjuncts may 

 be regarded as additions made to the revolving table to 

 make it available for the measurement of curvatures. 



