1882.] 



MICROSCOPICAL JOURNAL. 



85 



side the large frustules of diatoms e. g. 

 Navicula major and Surirella sj>len- 

 dida. 



It is not difficult to conceive how 

 these may, in some minute and early 

 form, have penetrated the frond of a 

 Closterium J it is not so easy to see 

 how they have entered an unbroken 

 frustule of a diatom, but the fact is 

 the same, and probably they had as 

 much connection with reproduction 

 in the one case as in the other. 

 Mr. Holland's observations are inter- 

 esting, and it is to be hoped that he 

 will some day be able to trace the 

 history of these, apparently desmid- 

 spores, in all its stages. 



In regard to Mr. Vorce's paper, it 

 is marked by careful, well-matured 

 statements, and the conclusions to 

 which he has arrived, are, in my 

 judgment, quite correct. 



I have not the least doubt that the 

 diatoms are enveloped by a mem- 

 brane, out of which the stipes, tubes, 

 etc., are formed ; my reason for this 

 opinion, indeed I may say the proof 

 of the existence of this membrane, 

 will be given in a paper upon the 

 " Life History of the Diatoms," which 

 will be published, I trust, before long. 

 In that paper I have also hinted at 

 an explanation of the movements, 

 based on experiment and observation, 

 but which, after all, may prove as 

 unsatisfactory as those mentioned by 

 Mr. Vorce. 



These movements, so curious and 

 so varied, are yet connected with the 

 structure of the frustule, and we must 

 not ignore this, in attempting to ex- 

 plain them, e.g. the nitzchise, which 

 have a continuous raphe, /. e., without 

 median nodule, or break, move in the 

 most lively manner, they are also long 

 and slender; the stalked forms move 

 when free, Cocconema for example, in 

 a long curve, Gomphonema straight ; 

 the navicula group move in straight 

 lines, but not in so lively a manner 

 as the nitzschiae. All these, except 

 the last named, have a median nod- 

 ule. The surirelleae, which have the 

 raphe along the four expansions, or 



al(z (two for each valve), move more 

 sluggishly, rolling over frequently, 

 and the amphiproreae and other 

 twisted forms, rock or twist as men- 

 tioned by Mr. Vorce, while' the cir- 

 cular forms, like Coscinodiscus, which 

 have the raphe probably all around 

 the margin of the cingulum, or con- 

 necting zone, and edge of the valve, 

 do not move at all, or if so, very 

 sluggishly. The movement then is 

 more or less regulated by the struct- 

 ure of the frustule, and in any ex- 

 planation we must not forget this. 

 The careful observations of facts 

 meanwhile should not be neglected, 

 and the publication of them, may 

 give the clue or hint that will guide 

 some other observer, possibly, to the 

 true solution of a phenomenon as mar- 

 vellous as it is at present inexplic- 

 able. 



Oblique Illumination, with a 

 Special Consideration of the 

 Capabilities of Immersion 

 Condensers, and a Note on 

 Symmetrical Illumination. 



BY ERNST GUNDLACH. 



The advantage of oblique illumina- 

 tion has long been acknowledged in 

 microscopical research. For a long 

 time the simple, easy, and common 

 method of oblique illumination with 

 the mirror seemed adequate for all 

 needs. Afterward, as objectives were 

 improved and their angle increased, 

 it was found desirable to have the 

 mirror swing out to the widest pos- 

 sible extent, to utilize the additional 

 angle, thereby gaining in resolution. 



But a little later, by the further 

 improvement of objectives, more 

 obliquity of illumination was de- 

 manded, and extremely thin stages 

 and other devices were employed to 

 get as near as possible to i8o°, then 

 the alleged limit of aperture. I say 

 alleged limit, because most people 

 then believed, and perhaps some now 

 believe, that to be the limit of the 

 theoretical angle of aperture. To 

 such, if there be any, I would say 



