BECK'S VERTICAL ILLUMINATOR. 219* 



ing on the action of the Beck illuminator, we coupled the- 

 instrument to my Tolles one-tenth and worked them 

 over Mr. Watson's Nobert test-plate, and in a very short 

 time succeeded in getting a most charming display of 

 the 19th band, and while the latter aperture was nearly 

 closed by interposing the circular edge of a large bull's 

 eye condenser which happened to be at hand on the 

 table. Mr. Watson and myself were both delighted 

 with the exquisitely beautiful display of this so-called 

 difficult object this 19th band. Other severe tests 

 were also taken in hand and resolved. Among others 

 we had a glorious show of podura under amplifica- 

 tion of some 4,000 diameters. It was demonstrated, 

 too, that the very best resolutions were only obtained 

 when more or less of the lateral aperture was closed by 

 interposing the circular rim ot the condenser, the clear 

 aperture left being in the form of -a crescent. Subse- 

 quently Mr. Watson devised the following described 

 attachment, which answers the purpose fully : 



The lateral aperture of the instrument is somewhat 

 enlarged and a hollow plug inserted therein, the opening 

 in this plug being the same calibre as the original aper- 

 ture. The plug is somewhat tapering, and fits the open- 

 ing "spring tight." It also projects outward slightly 

 beyond the circumference of the main tube. To the outer 

 end of the plug is fitted a little shutter, while a narrow 

 slit, about one-one-hunclred-and-fiftieth of an inch wider 

 is pierced through the shutter ; the whole so arranged 

 that the opening the plug can be partly or wholly closed^ 

 or the little slit used by itself; and when low powers are 



