HARDWICKE'S SCIENCE-GOSSIP. 



159 



much less attractive, having been conveyed by man 

 to considerable distances, by accident or design ! 

 Indeed, Mr. Bentham has told me that the Astrantia 

 itself was often an inmate of old cottage gardens ; 

 showing that it was not despised as an ornamental 

 plant ; in fact, though not a showy one, it is by no 

 means common-looking or unattractive. 



One more reason may be added to the above ; 

 namely, that the extent to which it has spread 

 throughout Stokesay Wood points to the time of 

 its introduction as being very remote. 



J. D. La TorcHE. 



Stokesay. 



MICROSCOPY. 



On an Ijipkoved Reflex Illujiinatob, for 

 THE Highest Powers of the Microscope, by 

 F. H. Wenham. — The above is the title of a paper 

 read by one whose name has become a household 

 word among microscopists, and all who use the 

 microscope owe him a deep debt of gratitude for 

 the many useful adjuncts he has from time to time 

 invented, and given to the public, without fee or 

 reward. Two of the best known are his paraboloid 

 and binocular prism. The former, as most of the 

 microscopical readers of S.-G. are aware, is used 

 for giving a black field, whilst the object is 

 brilliantly illumiuated, and few objects are more 

 beautiful than JPleurosigma formosum, Heliopelta 

 Metii, or Aulacodiscus formosus, when illumiuated 

 by means of the paraboloid. This method for obtain- 

 ing a black field is all that can be desired when no 

 higher objective is used than a \ ; but when the 

 highest powers came into more general use, it was 

 felt that the black field illumination to be used with 

 them was a desideratum. The author proposed 

 for this purpose a small truncated lens, attached to 

 the slide by means of oil of cassia or cloves, and 

 beneath this the paraboloid : this produced some 

 very good effects, but, to quote Mr. Wenham's own 

 words, it was a clumsy and unhandy combination, and 

 so difficult to manipulate that even in the hands of 

 experienced microscopists it sometimes failed to 

 show satisfactory results. The new illuminator 

 seems in every wiiy to be a great improvement on 

 the old plan, and if carefully made would produce 

 very satisfactory results. The illuminator consists 

 of a cylinder of glass, half an inch long, and y% in 

 diameter ; the lower convex surface is polished to a 

 radius of -^ ; the top is fl.at and polished, starting 

 from the bottom surface ; the cylinder is worked off 

 to a polished face at an angle of 6-1° ; close beneath 

 the cylinder is set a plano-convex lens of H focus. 

 The action of the cylinder will be understood by 

 referring to the following diagram (twice the 

 real size), a is the cylinder, b the lower plano- 

 convex lens, d;\d, f/rays of light reflected upwards 



from the mirror. If the two sides of the cylinder 

 had been parallel, the rays would have passed on 

 to e ; but as one of the sides forms an angle with the 

 base, the rays impinge on the sloping side, and are 

 reflected from it upon th^ flat surface at such an 

 angle as to suffer total reflection, ''s shown by the 



Fig. 108. Diagram of Reflex Illuminator. 



black lines (/) : if, however, a drop of water is 

 placed on the surface of the cylinder, and brought 

 into contact with the lower surface of a slide (c)j 

 the rays pass through, as shown by the dotted lines' 

 and are now totally reflected by the upper surface, 

 the black lines at g representing the reflected 

 rays ; but if an object, such as a diatom valve, or a 

 podura scale, is in close contact with the slide, it 

 becomes brilliantly illuminated, one portion of the 

 ray passing into it, the other being reflected j thus 

 giving a perfectly black field. The apparatus is 

 made to rotate on the focus as a centre. The 

 required objects on the slide are found by a low 

 power. The light is thrown up either by a plane 

 or concave mirror : the former is generally the 

 best and most controllable. Having got the best 

 effects, say on a diatom or insect-scale, by tilting 

 the mirror, we now proceed to rotate the illuminator 

 During this, the most exquisite unfolding of 

 structure takes place. The inventor then proceeds 

 to describe the superb appearance of the podura 

 scale when illuminated by this contrivance. He 

 says Amphipleura pellucida assumes a substantial 

 appearance not seen iu any other way, and at once 

 displayed its strife with au g that had never 

 resolved them before. In an addendum he describes 

 some experiments in order to ascertain, if possible, 

 the distance at which objects on a total reflecting 

 surface were rendered visible by light transmitted 

 iu consequence of their presence or contact thereon, 

 and from these experiments he inferred that no 

 light or colour effect would be produced if the 

 object was not within the distance of an undulation 

 that would produce colour; this would be less than 

 the ten-millionth of an inch. For further details 

 we must refer the reader to the June number of the 

 Microscopical Journal. — F. Kitton, 



