BECK'S VERTICAL ILLUMINATOR. 



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-hundred-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 



