MICROSCOPE AND OPTICAL APPARATUS 15 



very best objectives Were not until within a 

 few years at all suited to photographic pur- 

 poses, there being a decided difference be- 

 tween their visual and actinic foci, for the 

 former of which alone they were corrected. 

 The consequence was that the details of an 

 object showing sharp and clear upon the 

 focusing-screen were found to be somewhat 

 blurred in the negative. This applied only 

 to the lower powers, under a i", the difference 

 in the higher powers being so slight as to 

 require little or no attention. It was then the 

 practice, after obtaining a sharp visual focus 

 on the screen, to withdraw the objective very 

 slightly by means of the fine adjustment screw, 

 about ToVo" for the &" or i"; -sfa" for a "; 

 rfo" for the ii" objectives. This was not very 

 Satisfactory at the best, and a great advance 

 was made when Wenham advised that a double 

 convex lens be fitted in place of the back-stop 

 of the objective, one of 5-inch focus for the 

 A" and f", and one of 8-inch focus for the 

 ii" or 2" objectives. I have used for many 

 years a complete set of Beck's objectives 

 from 3" to -iV thus fitted, with entire satis- 

 faction, and mention these facts here for the 

 benefit of any who may be in possession of 

 these older lenses. They can readily have 



