418 Mr. Howard Gruhb [April 2, 



cemented on to a holder of this form, having (for smaller sizes) three 

 pieces of cork, to which the lens is attached, this holder being 

 screwed to a spud or nose on top of a post screwed to the floor. The 

 operator having applied the proper quantity of moist emery powder 

 between the grinder and the glass, proceeds to work the former over the 

 latter in a set of peculiar strokes, the amplitude and character of which 

 ho varies according to circumstances, at the same time that he changes 

 his position round the post every few seconds. 



****** 



Although, as I have shown, the harder material is abraded very 

 much more than the softer, yet the softer (the grinder) suffers consi- 

 derable abrasion as well as the glass, and the skill of the operator is 

 shown by the facility with which he is able to bring the glass to the 

 curve of the grinder without altering the curve or figure of the 

 latter. 



It is even possible for a skilled operator to take a lens of one 

 curve and a grinder of say a deeper curve, and by manipulation to 

 produce a pair of surfaces fitting together and of shallower curves 

 than either. 



Measurement of the Curves. 



In the early stages of grinding, gauges of the proper radius, cut 

 out of sheet brass or sheet steel, are used for roughly testing the 

 curves of the lenses ; but when we get to the finer grinding process, 

 it is necessary to have something much more accurate. 



For this purpose a spherometer is used. It is made in various 

 forms, generally with three legs terminating in three hardened steel 

 points, which lie on the glass, and a central screw with fine thread, the 

 point of which can be brought down to bear on the centre of the glass. 

 In this way the versed sine of the curve for a chord equal to diameter 

 of circle formed by these points is measured, and the radius of curve 

 can be easily calculated from this. 



I do not find the points satisfactory for regular work. They are 

 apt to get injured or worn, and for ground surfaces are a little 

 uncertain as one or other of the feet may find its way into a deep 

 pit. This particular spherometer has three feet, of about half an 

 inch long, which are hardened steel knife-edges forming three portions 

 of an entire circle. In using this it is laid on the surface to be 

 measured, and the screw with micrometer head is turned till the point 

 is felt to touch the surface of glass. This scale and head can then be 

 read off. The screw in this instrument has fifty threads to the inch, 

 and the head is divided into 100 parts, so that each division is equal to 

 sjyoo of an inch. With a little practice it is easy to get determinate 

 measures to y^^ of this, or -^-^q^ of an inch, and by adopting special 

 precautions even more delicate measures can be taken, as far probably 

 ^^ Ju-Q^j^-uu ^^ T^oV^iF ^^ ^^ inch, which I have found to be prac- 

 tically the limit of accuracy of mechanical contact. 



