224: Determining the Curvature of Spherical Surfaces. 



lies on the axis of the table. The scale-reading of an index 

 mark on the board is then taken and recorded. The mean 

 of several independent readings should be taken. 



In the case of some lenses the accuracy of setting is limited 

 by the departure of the surface from perfect sphericity. In 

 these cases it is possible to make the image remain stationary 

 in spite of the motion of the table provided the motion be 

 so small that the part of the surface at which the rays 

 entering the microscope are reflected is not near the edge of 

 the lens. A larger motion of the table causes a rapid motion 

 of the image. 



§ 9. The board is next adjusted to the a second position " 

 in which the axis of the table is a tangent line to the reflecting 

 surface. A small patch of lycopodium is placed on the surface 

 and then a sharp-pointed piece of wood is held against the 

 surface while the lathe-spindle is rotated so as to remove all 

 the lycopodium but a circular patch about a millimetre in 

 diameter. If the axis of the spindle be perpendicular to the 

 axis of the table, the tangent plane to the surface at the centre 

 of this patch will be parallel to the axis of the table. The 

 board is then adjusted along the scale so that the central 

 grains of the patch remain stationary when the table is turned 

 backwards and forwards about a mean position in which the 

 axis of the lathe-spindle is parallel to that of the microscope. 

 "When the adjustment is complete, the scale-reading of the 

 index mark is recorded. The mean of several independent 

 readings should be taken. 



The difference between the two mean scale-readings in the 

 first and second positions gives the radius of curvature of 

 the surface. 



To allow for the measurement of comparatively large radii 

 of curvature the board is provided with two index mirks, the 

 distance between them being 15 cm. When one mark is 

 beyond the range of the scale, the reading of the other is 

 taken. 



The following experiment serves as a test of the method. 

 Two lenses, one convex and the other concave, are selected 

 so that wide Newton's rings are seen by sodium light when 

 the marked face of the convex lens is placed in contact with 

 the marked face of the concave lens. The radius of the 

 convex surface is determined by the revolving table and the 

 radius of the concave surface is deduced from measurements 

 of the rings. The radius of the concave surface is also de- 

 termined by the revolving table and the two results for this 

 surface are compared. 



