WRIGHT: PETROGRAPHIC MICROSCOPE 467 



Tissot and Pellin 1 sought to remove the astigmatism by means 

 of a cylindrical lens placed above the eye lens of the ocular, but 

 this arrangement proved to be only partially successful. Recently 

 S. Becher 2 has studied the question in detail and suggests that 

 the astigmatism can best be obviated by converting the micro- 

 scope lens system into a telecentric system such that the rays pass 

 through the analyzer as parallel beams. He proposes to accom- 

 plish this by using objectives specially corrected for image plane 

 at infinity and to employ a weak positive lens above the analyzer 

 to focus the parallel beams emerging from the nicol on the image 

 plane of the eyepiece. 



Experience has shown that Becher's plan to remove astigma- 

 tism by means of the telecentric lens system is feasible ; but that 

 other arrangements in the optical system are better adapted from 

 a practical standpoint to accomplish the same purpose. 



In the first place it is not convenient to require a specially 

 computed objective for the observations in polarized light. Such 

 objectives would have to be computed and made specially, and 

 would serve only a special purpose, On taking up this matter 

 with the Bausch & Lomb Optical Company it was suggested to 

 the writer by Dr. H. Kellner 3 that a weak negative lens may be 

 used below the nicol in conjunction with an ordinary objective, 

 the negative lens to be of such focal length that the rays con- 

 verging toward points in the image plane are converted into 

 parallel beams and pass as such through the nicol. After emer- 

 gence from the nicol they are rendered again convergent by 

 means of a weak positive lens. The new plan was tried out 

 nearly a year ago and has been regularly introduced since then 

 on all research model microscopes constructed by the Bausch & 

 Lomb Optical Company. For observations in parallel polarized 

 light the new scheme works well and is satisfactory, but for 

 observations in convergent polarized light it is useless, as is also 

 the plan suggested by Becher. 



1 Comptes Rendus, 145: 866-877. 1907. 



2 Siegfried Becher. Uber den Astigmatismus des Nicols und seine Beseitigung 

 im Polarisations Mikroskop. Ann. d. Physik: 4 Folge, 47: 285-364. 1915. 



3 Letter of September 18, 1915. 



