526 



RECORD OF CURRENT RESEARCHES RELATING TO 



outline and midrib of the frustule, the objective being carefully cor- 

 rected. A few moments' gazing prepares the eye to perceive the fine 

 striaB which are then distinctly resolved, and, if the diatom be favour- 

 ably placed, over its entire surface. The light is managed to avoid 

 glare, and modified to suit the eye-piece, a somewhat dusky hue being 

 preferable and most conducive to distinctness. The beam admitted 

 through an aperture of any of the sizes figured is found sufficient to 

 amply illuminate the field of the G objective with the fourth eye-piece — 

 (x 1150) — an important consideration, as many valves will scarcely 

 yield to less amplification. 



One of the dry strewed slides, at present supplied by Wheeler, is 

 used by Mr. Williams, the specimens in which he is advised are of 

 fair average difficulty. Equally satisfactory results are, however, 

 obtained with slides by Hardman (Baker's), and other mounters ; no 

 valve resolved was in any instance specially selected, or " picked." 



But very few trials have as yet been made with specimens mounted 

 in balsam. Striae have, however, been certainly discerned on some 

 valves so mounted ; and observations will be continued as appearances 

 would seem to justify the anticipation of completely successful re- 

 solution. 



It is not pretended that there is any novelty in the use of a 

 diaphragm with the dry paraboloid ; but it is believed that a stop as 

 above described and figured has not been before applied to its cup. 

 Diaphragm or stop, however, resolves A. pellucida, a little experience 

 of their action demonstrating that in some respects the latter is occa- 

 sionally preferable to the former. It is further believed that there is 

 no previously published record of the resolution of A. pellucida by 

 means of the dry paraboloid, and the result exemplifies yet another 

 use (among the many) to which the Wenham dry paraboloid can be 

 applied, and its consequent great efficiency as an accessory to the 

 Microscope. 



Tolles's Opaque Illuminator.* — One of the most ingenious 

 methods of illuminating objects by means of light incident upon the 

 surface under examination with high powers has been devised by 



Mr. Tolles for Professor W. A. Eogers, 

 F.E.M.S., of Harvard College Observa- 

 tory, U.S.A. Professor Eogers has given 

 much attention to micrometry and the 

 preparation of diffraction rulings, and 

 in the examination of specimens of his 

 own rulings on steel, &c., he found the 

 need of " opaque illumination," which led 

 to the adoption of the following plan, 

 which he considers to be superior to any 

 other he has tried. 



In Fig. 45, A represents the front 



lens of a ^-inch objective (not to scale). 



P is a small and narrow prism made to 



side into the bra'^s mount of the front lens to about the position 



shown. Parallel rays can be projected throughout the prism in _^the 



* 'Eiii;!. Mech.,' xxxi. (1880) p. 135. 



Fig. 45. 



