Nov., 1915] Making a Photographic Objective 7 



the glass will then act as its own guide. A disk about one inch 

 in diameter and a half of an inch thick could be cut out in a 

 little over a half of an hour. At B Fig. 1 is shown one of the 

 uncut slabs and at C and D two that are about used up. Though 

 working rather slowly this proved quite satisfactory though 

 wasteful of glass as it cut a rather wide scarf, copper must be 

 used ; brass was tried but the wear was so great as to render it 

 almost useless while the copper shows almost none. 



As these disks are cut out they are not only cone shaped but 

 the edges are very rough so that the next operation was to 

 grind these to smooth and true circular disks. This was done 

 on a Wells tool grinder shown in Fig. 2, which was slowed way 

 down by placing a large pulley on the counter shaft. The glass 

 to be ground was held by cementing it with pitch onto a piece 

 of brass rod which in turn was held in the drawing collet of the 

 head A. A special wheel B, made by the Norton people for 

 grinding the rims of spectacle lenses, was used and the machine 

 slowed until the wheel would keep wet when running against a 

 sponge, C, resting in water. The glass disk was in this way 

 kept dripping and heating entirely prevented. The grinding 

 was then carried out just as with any other material and the 

 edge was made beautifully smooth and true in a few minutes. 

 The beauty of pitch as a cement for holding the glass is that a 

 slight heating will soften it so that the disk can be shifted to 

 any position and then a dash of cold water clamps it in place 

 and at the same time the pitch will slowly yield to the slightest 

 pressure so that in a few minutes the glass is entirely free from 

 strain. In manufacturing this sort of work is done with a 

 diamond and is of course done much more quickly. 



The disks were thick enough to make two lenses each so we 

 sawed them into two as illustrated in Fig. 3. A is an old pol- 

 ishing head upon which was mounted a pulley at one end and a 

 copper disk, B, at the other, the disk being held between large 

 washers. C is a cast iron box fastened to an arm, D, hinged 

 at E and kept pressed against the copper disk by a cord passing 

 over two pulleys on the ceiling. This made a most excellent 

 automatic feed. The glass to be split was fastened to a block of 

 pine with pitch and the wood held in the iron box, C, with 

 wedges. Number 40 Carborundum was used with plenty of 

 water and the glass was cut through faster than a power hack 



