Nov., 1915] Making a Photographic Objective 5 



compelled to make, and how we finally ground and polished our 

 lenses would be of general interest. These methods do not 

 pretend to be the best, nor those actually employed by the 

 manufacturer, but they do illustrate how a lens can be made 

 and how a little ingenuity will enable one if he has the standard 

 tools of a machine shop to carry out almost any kind of 

 experimental work. 



As a preliminary to this, a brief outline of the problem 

 before the lens designer may be of interest. A simple lens 

 consists of a piece of glass bounded by either plane or spherical 

 surfaces as these, except in large reflecting surfaces, are the only 

 kind that can be made with sufficient accuracy. Such a lens 

 would have a great many defects or errors and would be unable 

 to give a sharp image on the photographic plate unless stopped 

 down to a very small aperture. By changing the radii of the 

 surfaces, and the thickness of the lens, the designer can vary 

 these errors, but after all is said and done he can do but little 

 to improve the single lens. He then combines lenses of difi'erent 

 forms and of difiierent kinds of glass into a single objective, in 

 this way making the positive errors of some of the lenses balance 

 the negative errors of the others, until he arrives at a com- 

 bination which is more or less perfect according to his skill as a 

 designer. How this is accomplished is far beyond the limits of 

 this paper, so I will now proceed to the micchanical side of the 

 problem. 



The first consideration is the glass ; of course it must be what 

 is known as optical glass and its selection is really part of the 

 work of the designer. Optical glass is nothing more than a 

 very perfect kind of glass which has been exquisitely annealed. 

 You are all familiar with the intense green of window glass when 

 seen edgewise; a piece of white paper will hardly be changed 

 in color when seen through twelve inches of a good optical 

 crown. The best optical glass is not made in this country, but 

 must be purchased from either Schott & Gen. of Jena or Mantois 

 of France. The Jena glass has become very celebrated and 

 most of the lens makers state that their lenses are made out of it 

 and as a consequence most people think that Jena glass means a 

 certain kind, while, as a matter of fact, their catalogue for 1909 

 shows about seventy different varieties. These differ in 

 optical qualities and chemical composition, and cost from 

 about a dollar to five dollars a pound, with a few special varieties 



