LENS-HOLDERS 



249 



the lens should (by means of its joint) be placed horizontally. The 

 lenses now most suitable for such a holder are those constructed 



FIG. 200. Stephenson's binocular by Swift (1887). 



upon the Steinheil formula, composed of three cemented lenses 

 forming a system which gives relatively long working distances 

 with large flat field. As made by Zeiss they magnify 6, 12, 20, and 



