Nov., 1915] Making a Photographic Objective 11 



the lens as it is ground. The lens may be held in the hand or 

 cemented to a disk of brass having a center hole drilled in the 

 back in which is placed a pointed piece of steel held in the 

 hand, the lens being free to rotate about the pointed steel 

 holder. Of course where the lens has to be ground to a definite 

 thickness it must be held by hand. Flour of emery was used to 

 rough grind though coarser grades would have worked faster. 

 The final smooth grinding was done with a special fine emery 

 made for this purpose by Bausch and Lomb. Great care must 

 be taken in the grinding to keep the lens as nearly centered as 

 possible. A lens is said to be centered when the line which 

 joins the centers of curvature of the surfaces passes through 

 the center of figure. Obviously if a double convex lens could be 

 ground to a knife edge it would be centered but if this were done 

 the edge would be almost certain to crumble in the final polishing 

 and deep scratches result. The centering of a convex lens can 

 be watched by keeping the edge as nearly uniform of thickness 

 as possible with a concave lens, if the original blank is made 

 larger than necessary and care is taken to make the sides par- 

 allel, the centering can be watched by keeping a fiat edge of 

 equal width around the concave portion, the lens being^ placed 

 back on the flat tool, from time to time, as the work progresses. 

 If care is used the lens need be made but little larger than the 

 finished size to allow for the final accurate centering to be 

 described later. 



After being smooth ground the lens is beautifully smooth 

 and velvety to the touch but is just as much ground glass as 

 ever, that is, it is absolutely opaque. We now come to the 

 polishing. This is done with specially prepared rouge and 

 only an excessively small amount of glass is taken off. Lord 

 Rayleigh in a paper on "Polishing of Glass Surfaces" read 

 before the British Optical Convention held in 1905, states: 

 "I started with a finely ground surface, rather more finely 

 ground I think than is used in practice, and I found that in 

 order to obtain a pretty good polish it was necessary to remove 

 a weight of glass, corresponding to a depth of about 6 wave- 

 lengths. I do not pretend that such a polish would satisfy the 

 requirements of commerce; probably the 6 would have to 

 be raised to 10 or 12 in order to get to the bottom of the 

 deepest pits." When it is remembered that a wave length is 

 about the fifty thousandth part of an inch we realize how very 



