524 RECORD OF CURRENT RESEARCHES RELATING TO 



light from their oil-immersion condenser. The new objective has an 

 aperture, measured in crown glass, of 112^ (=1'27 numerical 

 aperture, nearly), which is stated to be the highest aperture hitherto 

 produced in Europe for a water-immersion. It is understood that 

 these opticians have also produced ^ and ^ objectives on similar 

 formulfe, and of nearly equivalent apertures. 



At the last scientific evening of this Society, Messrs. Powell and 

 Lealand also exhibited their new J-^ oil-immersion on Podura and 

 P. angulatum with central light from their achromatic condenser. 

 The aperture of the new objective, measured in crown glass of mean 

 index 1"525, is 110° (= 1*26 numerical aperture, nearly); the 

 covering-glass used was stated to be '004 inch, and the working 

 distance also • 004 inch, giving a clear object-distance of • 008 inch 

 from the plane surface of the front lens. In consequence of this 

 increased working distance the objective will be found far more con- 

 venient to use than the gV ^^ earlier formulae by these opticians. 



Zeiss's Adjustable Objectives. — These objectives, in which the 

 magnifying power can be varied from that of a 5-inch to a 2-inch lens, 

 have been further improved. In the original form the adjusting screw 

 pushed the hach combination upwards, which necessitated an alteration 

 in the diaphragm usually placed at the objective end of the tube. In 

 the improved plan the screw pushes the front combination forward so 

 that the objectives can be used with any stand. 



Durability of Homogeneous-Immersion Objectives.* — Professor 

 Abbe writes that Dr. Woodward's homogeneous-immersion and the 

 few others returned to Zeiss for repairs were " not damaged at all," 

 the oil it would seem having entered at the screw at the lower part of 

 the setting, and not at the edge of the front lens. Only two instances 

 are known in which the setting of the front lens became leaky, the 

 objectives being some of Zeiss's earliest productions. No homogeneous- 

 immersions now leave the works until the tightness of the front lens 

 has been ascertained by several prolonged exposures to oil. 



Professor Abbe considers that the durability of homogeneous- 

 immersion objectives is in no way inferior to that of other objectives, 

 and remarks that the cedar oil supplied by Zeiss does no injury to the 

 varnish of stands, or other brasswork, as shellac is not dissolved by it. 



Wenham's Dry Paraboloid and Amphipleura pellucida. — Mr. 

 George Williams has succeeded in satisfactorily resolving, on bright 

 ground, the transverse strice of A. pellucida with Zeiss's water- 

 immersion objectives (G | and K -^^), using as a condenser only 

 Wenham's ordinary dry paraboloid, having a diaphragm at its base, 

 or a stoji in its cup, such diaphragm and stop having respectively 

 an eccentric aperture. Both diaphragm and stop are made of sheet 

 metal. Figs. 40 and 41 being forms of the diaphragm ; Fig. 42, of a 

 shutter laid over the apertures (of Fig. 40) not intended to be used ; and 

 Figs. 43 and 44, of the cup stop. The figures are drawn to full size. 



The diaphragm and stop are not used in combination, but sepa- 

 rately ; and the forms figured have been adopted as giving the best 

 * ' Engl. Mccli.,' xxxi. (1880) p. 135. 



