SUPPLEMENT. 387 



which was designed by its inventor to give a dark field, 

 becomes, when used on balsam mounts, with American object- 

 glasses, having balsam angles ranging in the nineties, not a 

 reflex, but a direct illuminator, and a most efficient aid to the 

 definition of the lens. By it I am enabled to show the trans- 

 verse striae of No. 20 of the Holier plate so clearly that any old 

 lady who can read her family bible could hardly fail of recog- 

 nizing the striae on "this well-marked diatom." I have the 

 instrument with me, and shall be happy to show you its work 

 on the Moller plate. * 



It was mainly the purpose of these desultory remarks to call 

 your attention to the importance of instrumentation ; absolute 

 perfection has not been, and never will be obtained. There 

 never has been, nor will there ever be made, an instrument of 

 precision that does not embody inherently some radical weak- 

 ness, some dangerous fault. The practical astronomer is not 

 only early taught the nature and use of the various instruments 

 to be found in the observatory, and made acquainted with their 

 several imperfections, both optical and mechanical, but he is 

 compelled to acquaint himself thoroughly with the methods 

 employed to eliminate these imperfections. Having thus per- 

 force of his preliminary study acquired a thorough perception 

 of the Scylla and Charybdis which environ the use of instru- 

 ments, it becomes in turn a life's study to further remove exist- 

 ing difficulties, or to provide better and more competent means 

 of compensation therefor. 



I have no doubt but that it would be interesting and instruc- 

 tive to detail some of the exquisitely precise methods employed 

 by the astronomer to detect and compensate for the unavoidable 

 errors of instrumentation, but time forbids. I will, however, 



* Immediately after the introduction of the "reflex," mod-Scat ions, by- 

 changing the angle cf the facet, were made by the London opticians, so as 

 to adapt the instrument to their variously-angled objectives, similar modi- 

 fications were also made here in this country, by Messrs. Tolles & Spencer. 

 Three years ago the writer consulted a skilled artist with the view of mak- 

 ing the "reflex" in three separate mountings, changing the angle of the 

 facet in each. Nevertheless, the idea is Wenham's. The mere matter of 

 changing the angle of the facet would naturally occur to any one using the 

 instrument J. E. S. January, 1878. 



