ZOOLOGY AND BOTANY, MICBOSCOPY, ETC. 129 



to a hair's breadth by turning the tube, and any obliquity of the 

 pencil may be got either by inclining the Microscope or raising the 

 lamp. There is in this way no necessity for converting the lens into 

 a disk. The hemisphere sent out by Zeiss with his oil-immersion 

 lenses is, he thinks, too large. 



Dr. H. Van Heurck,* in a note in commendation of the diatome- 

 Bcope as an oblique condenser, says, " It has been said that oblique 

 condensers were of no utility through giving false images. I am not 

 of that opinion ; certainly, if object-glasses could be constructed so as 

 to resolve everything with an axial illumination (there are diatoms 

 much more difficult to resolve than Amphipleura pellucida), and if 

 these object-glasses could be supplied at so moderate a price that 

 every one could have them, then I should say that oblique con- 

 densers could be done away with. But, unfortunately, such is not 

 the case — at least, nowadays ; yet we may foresee that by discovery 

 of new media, our means of investigation may be considerably 

 improved. 



Meanwhile, the oblique condensers render notable services; in 

 cases where it is not allowed us to see, we may conceive. We know 

 that the valve of the diatoms is provided in most cases with alveoles, 

 and in others with fine punctuations. The strise, then, by the oblique 

 condensers are the solid and thickened parts of the valve, and the 

 distance between them allows us to judge of the size of the alveoles ; 

 also the direction of the striee shows whether the alveoles are disposed 

 in opposite series which produce longitudinal and transversal striae, 

 such as in Pleurosigma balticum, or in alternate series which produce 

 oblique striae, such as in P. angulatum." 



Oblique Illuminators. — The value of oblique illuminators has 

 recently been the subject of some controversy. Mr. E. M. Nelson t 

 considers that " oblique illuminators, be they diatomescopes, reflex 

 illuminators, revolver prisms, or what not, should, in the interest of 

 microscopical science, be consigned to the dust-bin. I am confident 

 that by their use only false images can be obtained. In former days 

 microscopists used this kind of illumination, and in consequence 

 talked of the striae on Pleurosigma, N. rJiomboides, &g. Now, we 

 know there are no striae at all on these diatoms ; the marks being 

 isolated dots, the striae owing their origin solely to the running 

 together of these dots by improper illumination. In the case of A. 

 jpellucicla, we have to content ourselves with the appearance of stri^, 

 simply because it is beyond the powers of our widest angled objectives. 

 I have not the slightest doubt that if an objective were made capable 

 of completely resolving it, it would appear similar to N. cuspidata, 

 having more dots to the inch transversely than longitudinally. Few 

 have worked with oblique illuminators more than I have ; my 

 experience leads me to say that an oblique illuminator for the 

 Microscope is not wanted." 



* Engl. Mech., xl. (1884) p. 365. 

 t Ibid., p. 242. 

 Ser. 2.— Vol. V. tt 



