NEW OIL IMMERSION OBJECTIVES, ETC. 315 



meiit, which adjustment he recommends be accom- 

 plished by manipulating the draw-tube to a possible 

 extent of two inches when working over covers varying 

 from 0.004 to 0.008; and here let the reader be re- 

 minded that these figures do not by any means express 

 the maximum variations of cover-glasses. 



o 



I have had an opportunity of critically examining but 

 one of these objectives, a one-eighth, imported by a 

 friend. This glass when worked with ten-inch tube 

 over my usual covering-glasses required downright 

 engineering of the draw-tube; furthermore, when 

 worked over covers 0.008 inch thick, it was impossible 

 to correct the glass at all, as the draw-tube of my 

 stand when fully closed would not decrease the distance 

 sufficiently to secure the proper correction of the ob- 

 jective. 



The performance of this one-eighth (when exactly 

 corrected) was very fine indeed, and closely resembling 

 the work of the Tolles or Spencer duplex. Superb as 

 its definition was, it did not excel that of my Spencer 

 duplex one-fourth a glass of half its nominal power. 

 The balsam aperture of the one-eighth was about the 

 same as that of my duplex glasses. 



During my first evenings with the oil immersion the 

 cedar oil behaved very well, but on the second and 

 third sittings it made me trouble enough. With the 

 tube inclined (as usual) the oil would most unaccount- 

 ably take a notion to run away; when this occurred, 

 any attempt to reinforce by adding more of the oil, 

 made things worse ; the only remedy was to clean the 

 slide nicely and commence operations again, de novo. 



