140 



Progress in Science. 



[January,, 



Mr. F. H. Wenham has succeeded in constructing objectives with a single 

 posterior as well as anterior lens, the only compound lens being the middle 

 combination. With respect to the fitness of the high angle of aperture of the 

 glasses at present in use for ordinary work, Mr. Wenham considers " that 

 a 2 V h of about 95 , accurately corrected, and having a long working distance, 

 say ^Vli of an inch, would be a valuable glass in the hands of a naturalist, 

 enabling him to see into things instead of a mere surface observation of a few 

 diatoms^ for the sake of performing the feat of defining the difficult marking 

 of some half-dozen of them, — and this is only what such a glass is at present 

 used for." 



Messrs. Powell and Lealand have constructed an objective of g l th of an inch 

 nominal focus ; its angular aperture is 160 ; the magnifying power is 4000 

 diameters with the A eye-piece, and it bears the B and C eye-pieces with no 

 other detriment than a slight loss of light ; it works well through a cover of 

 0-003 inch. It was exhibited at a recent meeting of the Quekett Microscopical 

 Club, and, notwithstanding the unfavourable conditions under which it was 

 tried, it showed the Podura scale sharply, without colour and with abundance 

 of light. 



At the December meeting of the Royal Microscopical Society Mr. Gayer 

 exhibited a micro-spectroscope of novel construction. The slit is placed in the 



lower part of the body at about an inch 

 Fig. 2. distance above .the object-glass ; the 



slit is adjusted by the usual con- 

 trivances, and a small right-angled 

 prism, B, and mirror, c, supply the 

 means of obtaining a second spectrum 

 for comparison. The image of the slit 

 is formed by the collimating lens, d, 

 above which are mounted two prisms 

 of 6o = , e and f. Attached to the curved 

 tube containing the prisms is the tele- 

 scope, g, having all necessary arrange- 

 ments for focussing, and also a micro- 

 meter, h. When the object to be 

 examined is so small that there is any 

 doubt as to its image filling the slit, 

 the spectroscope is removed, and the 

 ordinary draw-tube with erector substi- . 

 tuted ; the slit can then be viewed, and 

 so adjusted as to include the whole of 

 the object. The prisms are of such 

 dispersive power as to distinctly split 

 the D line. The points of novelty are 

 the position of the slit and the employ- 

 ment of a telescope to view the spec- 

 trum, which of course allows of varia- 

 tions of magnifying power by changing 

 the eye-piece. Mr. Gayer claims for 

 this micro-spectroscope the advantages 

 of increased light and greater dispersion 

 than in the ordinary direct vision in- 

 strument placed over the eye-piece. 

 The latter property is not altogether 

 an unmixed gain, for although dispersive power is invaluable for separating the 

 bright lines of incandescent gases, the conditions required for the work of the 

 micrc-spectroscope are very different; the majority of absorption-bands are by 

 no means sharp or well-defined at their edges, and are, as a rule, best seen 

 with prisms of comparatively low dispersion, as more powerful instruments 

 only thin out the bands and render their boundaries less evident. 



