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The Ohio Journal of Science [Vol. XVI, No. 1, 



spherometer. This spherometer we were compelled to build as 

 we could find none of sufficient accuracy on the market and it is 

 described in a note at the end of this article. 



In Fig. 4, R is simply a steady rest made with the large 

 overhang to allow the slide B to swing under it in turning a 

 convex surface. Two master laps, male and female, must be 

 made and carefully ground together. Every effort should be 

 taken to make these as accurate as possible since upon these 



Fig. 5 



depends the goodness of our lens. This special tool is easy to 

 make and leaves nothing to be desired in its operation. Detail 

 drawings and directions for making it are given in a note at 

 the end. 



We now come to the grinding or lapping of the lenses them- 

 selves. This is done in a lap turned as above and carefully 

 fitted to the master laps and which must be trued from time to 

 time as the work progresses. This lapping of glass is entirely 

 different from the lapping of metals in that, while in metals the 

 lap is to be kept almost free from the abrasive, in glass the lap 

 •must be freely supplied with emery and water or deep scratches 

 will result. The best way to apply the emery is with a paint 

 brush; the brush, satviratcd with emery, being held in front of 



