302 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



glass, as nearly as possible), and the working distance is then • 024 inch 

 — probably the greatest working distance hitherto obtained with a 

 yV of that aperture, a result of course due to the homogeneous- 

 immersion formula. 



The new objective thus affords an important practical demon- 

 stration of the accuracy of the views enunciated by Professor G. G. 

 Stokes, in his paper in vol. i. pp. 140-1, that if a front lens, 

 aplanatic per se, were constructed of very large aperture, by reducing 

 the thickness of the lens zones of the aperture would also be cut off, 

 and the distance gained between the spherical refracting surface and 

 the focus would be available for " working " distance. The three 

 fronts exemplify this exactly, for they are all made from the same 

 substance of glass and the curvatures have the same radius, the 

 interchange of fronts simply changes the aperture and the working 

 distance, the magnifying power being the same with each (allowance 

 made for the impossibility of moimting the fronts, so that the vertices 

 of the curved surfaces should be exactly at the same plane — that is, 

 at the same distance from the posterior combinations). The utilized 

 diameter of the posterior lens (with the front of highest aperture) is 

 the largest that has been hitherto utilized for a ^V ; the second front 

 utilizes a less diameter, and the third still less, — the ratios being 

 that the front of lowest aperture utilizes a diameter expressed by the 

 numerical aperture 1 • 0, the second one 1 • 28, and the highest 1 • 43, 

 which can be actually verified by the help of an auxiliary Microscope. 



Murray and Heath's Polarizing Apparatus. — Mr, E. C. Murray 

 has devised a plan of applying the analyzing Nicol in a sliding-box 

 fitting, to be attached to the microscope-tube as a nose-piece, by which 

 means the prism can be readily shifted in or out of the field for 

 examining rock-sections, &c., by ordinary or polarized light. 



Fig. 48. 



Fig. 49. 



Fig. 48 shows the device (f size) ; A is the " Society " screw 

 for attachment to the tube, while the objective is screwed into D ; B 

 is the sliding-box, the tube C carrying the Nicol's prism, that can be 

 rotated by the double-milled edge shown. When the analyzer is not 

 required it can be pushed " home " in the box, and the other tube then 

 leaves the axis of the Microscope free. A spring at the side slightly 

 keys the sliding-box in the two positions required, that is, either using 

 the Nicol or not. The polarizing prism (Fig. 49) is mounted in a 

 somewhat similar manner for use beneath the stage, the milled edge 

 below giving the rotation. 



