JiTjE, 1902.] 



KNOWLEDGE 



141 





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Conducted by M. I Cep§s 



PoSD-i.iFE Collecting in Junk. — The months of April 

 and May have been so abnormally cold this year that it is to be 

 expected many of the pond organisms which usually make their 

 appearance in May will have been retarded, and will only come 

 on in June. There are, hawever, summer forms which hardly 

 ever occur earlier than June, and the most interesting of these 

 amongst Rotifers is Pedalion ruiriitii, with its si.\ arthropodous 

 limbs : Si/nc/i(Ftu stylata, with its long-spined floating eggs, and 

 Si/richirla qrandi<, the largest species of this genus, may also now 

 be looked for in lakes and water reservoirs, as well as the rare 

 free-swimming FlosritlarM peltujica. In the same waters will 

 be found two free-swimming colonies of Yorticella : Ejjisli/lis 

 roliiiu and Zoothamniuin limnelifum. In June it often happens 

 that certain wator-fleas, Daphnia and Bosmiua, also Cyclops and 

 their larva?, increase to such an extent as to render the existence 

 of free-swimming Rotifers almost impossible in these waters, and 

 the latter consequently quickly disappear, though they maj' have 

 been swarming a few weeks earlier. In ponds, however, where 

 this does not occur. Rotifers of many genera may be found, and 

 attached to submerged water plants Lacinularia !>ociaUs and 

 Meijalolriicha allio-jiarii-aiis should be looked for, whilst in reedy 

 ponds the free-swimming spheres of Cnnochihis rolvox may 

 occur. Mossy pools, in addition to their special rotiferous fauna 

 of I'hilodinii , Cntlhliiio-, Adlnetti. Cuthi/jyim, Di.<ti/lii, and .I/oho- 

 «ty/<i, will also contain waterbears and shelled Rhizopods such as 

 DIHtujia and ArcflUi and numerous free-swimming Infusoria. 

 Polyzoa, such as PltiniateUa repens. FredericcUa sulUina, Luph'ipus 

 cii/slalliniis, and CrhhitcUa mucedo, though not common, should 

 be abundant in suitable localities. 



CovER-f;r..\ss Correction. — With dry microscope objectives 

 of considerable numerical aperture, the thickness of the cover- 

 glass through which an object is examined is found to seriously 

 affect the definition of the image : an objective which is quite 

 satisfactory with some definite thickness of cover-glass showing 

 decided spherical aberration in the sense of under-correction 

 when the cover-glass is thinner, in the sense of over-correction 

 when the cover-glass is thicker, than the standard adopted by 

 the optician. This spherical aberration results from the refrac- 

 tion of the raj-s in the plane surfaces of cover-glass and objective- 

 front respectively, and is negative for the cover-glass suiface, 

 positive in the front lens-piano, the latter preponderating owing 

 to the greater diameter of the cone of rays from the object 

 where it enters the front lens. 



With the right thick.iess of cover-glass, the remaining under- 

 correction is exactlj' balanced by an equal over-correction in the 

 lenses of the objective ; but a thin cover-glass ]>roduces insuffi- 

 cient over-correction (the diameter of the cone of rays being 

 too small where it meets the surface of the cover-glass) ; a thick 

 one too much. 



As it is not practicable to strictly adhere to the standard 

 thickness of cover-glass adopted by the makers of an objective, 

 it becomes necessary, at least in critical work, to compensate 

 for the effect of the cover-glass, and this can generally be done 

 in two ways, viz. : — 



1. The tube length is found to affect the spherical correction, 

 lengthening of the tube introducing over-correcti<m, shortening 

 of the tube under-correction. Hence, we may generally secure 

 good definition with objectives in fixed mounts, even when 

 tlie thickness of the cover-glass differs considerably from the 

 standard, by adjusting the tube length, thick covers requiring 

 a shorter, thin ones a longer, tube. It maj-, however, be 

 mentioned that some of the relatively cheap " achromatic '' or 

 '■ semi-apochromatic " objectives by leading Continental makers 

 form a curious exception ; they give very good definition with 



their standard cover-gla.ss, but no alteration of tube length 

 seems to enable one to get anything like the same definition 

 with cover-glasses of considerably different thickness. 



2. With objectives of short focus the range of tube length is 

 hardly sufficient for the purpose of compens;ition ; for these 

 the adjustable mount— "correction-coUar'' — is a very desirable 

 and useful arrangement, which acts by altering the distance 

 between the front and back len.ses of an objective. A reduction 

 of this distance introduces under-correction ; an increase, over- 

 correction, and a scale on the " collar" serves to indicate the 

 position of the lenses. 



>Iost English opticians seem to adhere to the old plan — very 

 convenient and inexpensive to them, but puzzling to the micro- 

 scopist — of an arbitrary scale ; the German plan of carefully 

 marking the scale so as to show the thickness of the cover-glass 

 in l/10l)ths of a millimetre seems ever so much more convenient 

 and useful. 



With an objective having a scale thus divided and an object 

 under a cover-glass of known thickness, the use of the collar is 

 simplicity itself ; we simply set the collar to the division of the 

 scale corresponding to the thickness of the cover-glass, and the 

 tube length to that stated by the maker, and the instrument 

 will at once give its best defiuitiou. 



But generally these ideal conditions do not exist, and then 

 the adjustment of the tube length, or of the correction-collar, 

 becomes a delicate and difficult operation, which miy, however, 

 be accompbshed in a systematic manner, if we bear in mind that 

 we are aiming at the elimination of ''spherical aberration.'' 

 The latter may be defined as a difference of focus between the 

 central and marginal zones of an objective. Hence the correct 

 tube length or the correct position of the correction-coUar has 

 been found when some strongly-marked detail or outline of the 

 object remains in exact focus under anj' change of illumination, 

 say from a small to a large diaphragm oi)ening, or, better still, 

 from central to very oblique light, these changes being of course 

 made very carefully so as not to disturb the other adjustments. 



The following mudux operandi^'Wl be the safest and quickest. 

 Sfcirt with the sliortest tube length, or when there is a correction- 

 collar with the position corresponding to the thickest cover- 

 glass, carefully focus some sharp outline with, say, a \ central 

 cone ; then change to a J cone, or, still better, to very oblique 

 light. Unless the object — owing to an exceptionally thick cover- 

 glass or a very badly-adjusted lens — is beyond the range of j-our 

 adjustments, you will find evidence of under-correction, that is, 

 the lens will have to be brought closer to the object with the 

 wide cone, or oblique light, than with central light. 



Gradually lengthen the tube, or turn the collar, repeating the 

 above observation after each changing until all evidence of 

 .spherical aberration has disappeared ; the instrument is then in 

 correct adjustment uilhin your mni limits of rision. 



It is advisable to start with the adjustment corresponding to 

 the thickest cover for the simple reason that this lessens the 

 danger of running through the cover-glass and destroying the 

 object and possibly the front lens of the objective when dealing 

 with a lens of a short working distance. 



Pkactical Sche.me. — A reader — Mr. J. Searle, of Oporto — 

 has kindly sent a considerable number of centipedes of an 

 unusual kind, which I have so far been unable to get identified, 

 for distribution among readers of Knowleixje. Unfortunately 

 they have been sent in a dry condition, and very few of them 

 are, in consequence, whole, but, with care, very interesting slides 

 can be prepared of the various parts, and especially the legs, 

 which appear to be the most interesting feature. 



I am hoping, within the next few weeks, to receive a further 

 quantit)' from this gentleman, preserved in .spirits of wine, and 

 shall be pleased to send a specimen to a limited number of readers 

 who apply for same, enclo-ing a stamped addressed envelo|>e. 



The method of treatment for these insects would be as 

 follows : — Place them in water for three hours, transfer to ten 

 per Cint. of caustic-potash in distilled water until they become 

 soft ; this will probably take some days. When soft, pour away 

 the potash and add water, which must be changed several times 

 until all tho jiotash is washed away. Pour away the water and 

 add concentrated acetic acid and soak for twelve hours, or until 

 a convenient time for continuing the process, when they should 

 be transfen-ed from acetic acid to water and soaked for half an 

 hour. Lay out in the manner desii-ed, and after once more 



