Nov., 1915] Making a Photographic Objective 



9 



carriage, was mounted the sphere turning rest. This consists 

 of a base. A, in which the shde, B, is so mounted that it can be 

 rotated about the center, C, by turning the milled head, D, 

 which carries a worm at the opposite end. E is the tool post 

 with the cutting tool T and L the lap to be turned. A hole 

 was drilled at C into which was fitted a round piece of steel the 

 upper end being pointed and then half cut away like a center 

 reamer. This was used in finding the zero; the rod, pointed 



Fig. 4 



end up, was placed in the hole at C and the cutting tool adjusted 

 against the flattened side. The zero position is then deter- 

 mined by measuring, with an inside micrometer, the distance 

 from the tool post to a stop placed at the end of the slide B. 

 By adding to or subtracting from the zero reading of the 

 micrometer the length of the radius of the grinding lap, the tool 

 post may be set to the proper position for either a convex or a 

 concave surface. This, however, is only approximate, for these 

 laps must be made with the highest possible accuracy. After 

 sufficient cuts have been taken to give a spherical surface, the 

 radius is carefully measured with a special spherometer and 

 the error in the radius corrected by changing the position of the 

 cutting tool by an amount calculated from the readings of the 



