RECKKATIVK S'MK' 



31 S 



bullVoyu lens, and this might enable a* to 



rately the structure of such unapproachable finene** a* obtain* 



in Amphipleura, peUucida. The direct light of the sun. when 



reflected by tho plane prism, 



would thus be represented by a 



very close approximat 



" In the mechanical adjust- 

 ment of the prism to the sub- 

 stage, I would suggest a cradle 

 above a ball-and-sockot joint, 

 jn prism* are often mounted, 

 with t'n- .I'i'htioii of a jointud 

 arm, an used for the cxt<'n.-uim 

 of tin 1 mirror of our microscope 

 sideways, and, if necessary, a 

 i-huiiping-sorew to keep tho 

 prism in position. At present, 

 I fix tho prism on the sub-stage 

 with an indiu-rubbor band. All 

 that is required is the power 

 of turning the prism on its 

 axis, and also of placing it 

 over any diameter or any chord 

 of tho sub-stage. In the latter 

 position, tho prism lying over a chord from 30 east of the 

 vertex of the stage to 30 west of south, and its face slightly 

 inclined to the upper stage, very effective obliquity is obtained. 



The lamp, of 

 course, stands 

 to tho west. We 

 must rotate the 

 valve by the 

 circular motion 

 of the upper 

 stage, till the 

 hemispheres are 

 not obscured by 

 the parallel lines 

 of their own 

 shadows. When 

 they reach their 

 proper place they 

 seem to start 

 into existence, 

 and the degree 

 of elevation is 

 conferred per 

 sallum. By this 

 perfect com- 

 mand over its 



movements, tho ' diatom-prism ' (thus named from its first 

 application) will meet every requirement for oblique, direct, 

 and dark-ground illumination, while its simplicity and indepen- 

 dence of harness, in the 

 shape of diaphragms or 

 stops, is a chief charac- 

 teristic. The light being 

 nearly parallel, the prism 

 may be moved, by the rack- 

 work adjustment of the 

 sub-stage, to a considerable 

 distance below the object 

 without materially weaken- 

 ing tho illumination, and 

 the slight diminution of 

 light thus obtained is ad- 

 vantageous when using low 

 powers. 



" It is impossible to avoid 

 noticing the remarkable 

 stereoscopic effect of this 

 parallel reflected light. On 

 a Barbadoes slide, for in- 

 stance, the objects are seen 

 under an inch power, and 

 on a dark ground in very striking relief ; 

 effect is remarkably visible when viewing 



of muscular fibre i* also well displayed, new beauty is 

 in the Podora scale, and infusoria and portions of 

 may be examined with additional interest. 



" It seems to be owing to 

 this stereoscopic effect of 

 parallel light and natural 

 shadows, that the hemispheres 

 of diatom-valves are seen be- 

 yond all doubt as elevations. 

 We seem to be looking at an 

 opaque body illuminated from 

 above, and the appearance in 

 the microscope is exactly 

 similar to a model, made to 

 scale, in plaster of Paris. On 

 the other hand, when we have 

 anything approaching to de- 

 pressions, as in the markings of 

 Trictratium and Itthmia, these 

 depressions are, as it were, 

 palpably felt. The hexagonal 

 marking* in Trictratium are 

 of special interest. At every 

 angle of the hexagon there is a 

 hemisphere of larger size, and smaller hemispheres, in contact 

 with each other, form the sides, so that it is questionable 

 whether the depression is deeper than the radius of the hemi- 

 spheres them- 

 selves. A simi- 

 lar inquiry aUo 

 presents itself 

 when viewing 

 the irregular 

 though some- 

 what circular 

 markings form- 

 ed by an arrange- 

 ment of small 

 hemispheres on 

 the surface of 

 Isthmia. 



"I felt un- 

 willing that the 

 present session 

 should close 

 without giving 

 some account of 

 my observations 

 to those who 

 have more lei- 

 sure than myself for pursuing these interesting researches." 

 (Transactions of Royal Microscopical Society.) 



And now let us ask, What were the pictures that formerly 



represented the curious 

 figures brought out by Mr. 

 Reade'a illuminator, and 

 delineated in Figs. 4o, 44 

 in this paper ? They are 

 shown in the next engrav- 

 ings (Figs. 9, 10, and 11), 

 copied from " Reade's Prism 

 I for Microscopies Illumina- 

 tion," by Samuel HigbJey, 

 F.Q.S., which represent tho 

 different appearances tha, 

 seemed to embody reality 

 to the minds of tho various 

 microscopical observers and 



A'A""'*^ ^rf^^^^ > stf > B draughtsmen, but which, in 

 .^^ H" v, reality, convey a wrong 



of the actual 



and 



the 



tho same 

 proboscis 



of the blow-fly on a light ground. The peculiar character 



Highley, in commenting 

 graphically upon this step 

 in microscopical investiga- 

 tion, says: " Pleurorigma angulatum, first discovered in 

 1841, on confervas in the Humber, at Hull, was carefully 

 examined by Sollitt and Harrison. With a i-inch objective, 



