884 RECORD OF CURRENT RESEARCHES RELATING TO 



Tlio tube is suspended from two parallel arras, whose terminal 

 points are movably connected with both the pillar of the microscope 

 and the tube itself. These arms with medium adjustment are exactly 

 horizontal ; but if the tube is raised or lowered by means of the micro- 

 meter-screw, which acts upon a projecting piece between the arms, the 

 latter assume a slightly oblique position. The movement effected by 

 means of the screw corresponds therefore to the displacement of a 

 parallelogram of which one side remains fixed in a vertical position 

 while the opposite side is slightly raised or lowered, still preserving 

 the parallelism. Since the displacement takes place between the 

 points of eight screws, any shifting of the image is entirely avoided, 

 and friction is reduced to a minimum. In consequence of this the 

 micrometer-screw turns very easily, and dead-way is avoided with 

 equal resistance on both ends.* 



Construction of Immersion Objectives-t — The following note is 

 by Mr. Wenham, and we therefore give it verbatim (with the wood- 

 cut), though we are not sure that we altogether understand what is 

 meant to be conveyed, especially in regard to dispensing with the 

 use of oil. No alteration of the front of an objective can, as it seems 

 to us, ever make a water-immersion objective equal in aperture an 

 oil- (homogeneous) immersion : — 



" From the above | it may be inferred, that if the front of an 

 object-glass, in cases when the aperture is supposed to be limited by 

 water from rays reflected back and increased by an intermedium of oil 

 of cedar or cloves, if the first surface is also made concave, it would be 

 the means of dispensing with the objectionable use of oil. I have 

 tried some experiments this way. Fig. 102 is the front lens of an 

 p ,f.n immersion -jIq -inch object-glass. At first the con- 



cave surface of the front was made much shallower 

 than is shown, without any appreciable difference 

 in effect from that of a flat plane. The concave 

 was then deepened till it reached to^near three 

 times that of the hemispherical back radius, with 

 a slightly improved result in the way of increase 

 of light and flatness of field. The experiment was not carried further. 

 The radius of the back convex is "045, that of the concave "13. Of 

 course if oil of the same optical properties as the glass were to be 

 used, the effect of the concave surface would be simply nil. It would 

 then act like a flat front." 



Mounting of the Front Lens of Immersion Objectives. — Messrs. 

 Powell and Lealand claim to have made a water-immersion ^, with 

 a numerical aperture of 1 • 30 or 155° (the theoretical maximum being 

 1-33 or 180°). 



To obtain this aperture the plan described by Professor Abbe § is 



* Nageli and Scbwendener, ' Das Mikroskop,' 2nd ed. 



t ' Am. Mon. Micr. Journ.,' i. (1880) p. 101. 



% A description of the construction of the immersion-paraboloid. 



§ See this Journal, ii. (1879) p. 821. 



