Nov., 1915] Making a Photographic Objective 13. 



falling to the bottom, and leaving clear water above the precip- 

 itated rouge. The upper two-thirds of the rouge will be almost 

 perfect and will give a beautiful polish when carefully siphoned 

 off. This should be kept in tightly corked bottles, one of the 

 best things is a horse radish jar as this has a place for the 

 handle of the brush in the glass stopper, and all dust and grit 

 can be easily washed off before the jar is opened. For polishing, 

 the lens is cemented to a handle at whose end is a piece of brass 

 turned to fit the lens in the sphere turning machine already 

 described. Even in a small lens the polishing tool must be run 

 slowly, the speeds of our machines run from 170 to 300 revolu- 

 tions per minute ajid the fastest can seldom be used. The 

 reason of this is that the lens fits the polisher so perfectly that 

 almost a perfect vacuum is formed and the lens hugs the pol- 

 ished so closely that it is impossible to hold it in small sizes by 

 hand alone and in the case of a convex surface, if the cavity is 

 carried clear out to the edge of the glass disk, this may be broken 

 simply by the friction due to this grip of the glass and pitch. 

 Fig. 5 shows a horizontal polishing head at B and a vertical one 

 at C. There is little choice except that for convex surfaces B 

 seems the best, as it can be run faster, while for concave C seems 

 better. 



The lenses are now ready to be centered, that is, the circum- 

 ference so turned that the line which joins the centers of 

 curvature of the two spherical surfaces shall pass through the 

 center of figure. In order to accomplish this, the lens is first 

 cleaned from the pitch used to cement it to the handle used in 

 holding the lens for polishing. For a long time I could find no 

 way of doing this satisfactorily when pitch was the cement; 

 finally, I laid my troubles before Dr. A. M. Bleile, Head of the 

 Department of Physiology, and he suggested to first soak the 

 lens in lard and then wash it in benzol (CeHe). This worked 

 like magic though the first time I tried it I used some lard that 

 had been heated with some pitch in it which made the lard very 

 soft in fact almost as soft as it could be and yet not be an oil, 

 and this same lard was used over and over again. The action 

 is rather peculiar; the lard does not apparently effect the pitch 

 at all but after a few minutes in the benzene it all flakes off and 

 leaves the lens perfectly clean. The actual centering is then 

 carried out on the grinding machine shown in Fig. 2 ; A holder, 

 D, whose front face has been turned in the spherical turning 



