August, 1903.] 



KNOWLEDGE. 



189 



sides are cemented in the fire by means of a fusible glass 

 cement, and thus the trough is and remains watertight. The 

 trough (h) is also a useful type but it is not reversible, will 

 not stand by itself on the table, and, being cemented with gold 

 size or marine glue, is liable to leak. The troughs usually sold 

 are semi-circular in shape, a very bad type, because in addition 

 to the above defects, the least amount of tilting on the stage 

 will cause the water to run out over the edge. Thicker troughs 

 are objectionable because the substage condenser cannot work 

 through them, and the animals cannot be properly illuminated, 

 though sometimes such troughs may be required by the size and 

 nature of the object. 



A few words on pipettes will not be out of place here. The 

 old-fashioned way of using the finger on a straight or curved 

 glass tube to capture Pond-life is si unsatisfactory that I have 

 been driven to invent new pipettes for more precise and exact 

 work. Fig. '.-> (c, (/, and e) represent the i)ipette3 in constant use. 

 (c) is a glass tube about j% inch in diameter and 8 inches long, 

 which tapers from the middle to a point more or less fine, 

 according to the size of the animals one wishes to capture. 

 Over the wide end is placed an indiarubber teat, by means of 

 which any single specimen, or scores of animals, can be sucked 

 up with the least quantity of water, (d) is another type of 

 pipette, having a still finer action : it is 6 inches long, funnel- 

 shaped at one end, and tapering gradually from the funnel to a 

 fine point ; the funnel is J irch wide, and covered with an 

 indiarubber membrane. {/■) is a similar, but smaller and finer 

 pipette, 3.} to i inches long, f )r picking up small Rotifers in a 

 fraction of a drop of water under the dissecting microscope. 

 The slightest touch on the membrane is sufficient to expel or 

 bring in the water, so that one has complete control over the 

 amount of wat jr that is taken up, and there is much less risk of 

 losing the animal one wishes to transfer to the compressor. 



The old-fashioned live box with rai<ed tablet, still largely 

 sold with microsco])es, is quite useless for Pond-life, for the 

 simple reason that the objects cannot be properly illuminated 

 with the substage condenser. This consideration led me long 

 ago to design the live box ( /), in which the glass tablet is fixed 

 flush with the brass plate, and is of small size, thus leaving a 

 wide ring all round. This arrangement allows all objects on the 

 tablet to be perfectly illuminated from below by the achromatic 

 condenser, both with transmitted light and under dark ground, 

 and at the same time they can be reached and followed from 

 above with both low and high powers, and oil-immersion 

 lenses, to the very edge of the tablet, and wherever they may 

 wander. For more exact woik, when it is desired to hold a 

 single Rotifer between the two glasses and prevent its wandering 

 about, I have devised the compressor (f/), in which the pressure, 

 and the thickness of film of water, can be accurately regulated 

 by a screw acting against a spiral spring. At the same time, 

 water, or reagents, can be added, if desired, without raising the 

 cover. When properly and well made, this compressor works 

 exceedingly well, and I have had it in constant use for years, 

 but some makers, unfortunately, have introduced variations and 

 so-called " improvements " which just take away some of the 

 essential and useful points. The semi-circular thin cover-glass 

 must be cemented to the underside of the brass ring with a 

 little gold size, so as to be quite firm and rigid, otherwise its 

 action becomes uncertain, and very small objects cannot be held 

 fast, or else are suddenly crushed. 



Some more simple apparatus and devices may be mentioned 

 for cases where no live box or compressor is at hand. An 

 excavated glass slide makes a fair live cage ; a drop of water 

 containing the animals is placed in the c;ivity so as to just fill it 

 and no more ; anotlier drop of clean water is placed by the side 

 of the cavity, and a clean thin cover-glass is lowered on to that 

 second drop ; then, by means of a needle, the cover is slowly 

 pushed across the cavity, which can thus be covered without 

 enclosing an air bul)l)le ; the superfluous water is taken up by 

 blotting paper, the cover-glass being held in position by capillary 

 attraction. This forms a good slide for low and medium powers, 

 but not for high powers. Another good temporary slide can be 

 made by placing three small fragments of No 1 thin cover-glass 

 near the middle of a glass slip in form of a triangle ; the drop 

 of water containing the animals is placed in the centre, and a 

 clean thin cover-glass is lowered on to the drop so as to rest on 

 the three glass fragments, which prevent the animals getting 

 crushed. If there be too much water it can be removoil with 

 blotting paper. Low and high powers can be used on this slide 



as far as the movements of the animals will permit, but not 

 oil- immersion lenses. 



Having thus mentioned some essential and neoessarj' apparatus, 

 I will close with a few remarks on the examination of living 

 Pond-life. The free-swimming organisms, including such forms 

 as Volvox ijlobatur, collect on the light side of the window 

 aquarium, and can there be ])icked up in small or large numbers, 

 and quite clean, with the large pipette, and placed in the trough; 

 or any particular species can be selected with the aid of the 

 tank microscope, and taken up with the smaller pipette, and 

 transferred to the live box or comjjressor in a single drop of 

 clean water, both hands being free for this operation. 



The fixed forms, such as Polyzoa, Slephanoceros, Afelicertu, 

 FloscuUs, etc., amongst Rotifers, and Ste.ntor, Carrhi'.sium, 

 Zoothamniitm, etc , amongst Infusoria, require a little manage- 

 ment. If simply placed in a trough, these are often obscured, 

 or incapable of being properly illuminated, by being too crowded, 

 or by part of the weed over or underlying the objects, and also 

 by floating particles in the water. The best result is obtained 

 by trimming, that is by cutting off a very small piece of weed 

 or leaf on which the animal is attached — in a watch glass under 

 the dissecting microscope if necessary — and then transferring it 

 with the pipette to the compressor into a drop of clean water ; 

 it can there be arranged with a needle or bristle as may be 

 desired, and after lowering the cover-glass, fixed and held fast, 

 at the same time giving the animal perfect freedom to expand. 

 In this position the animals can be reached with the achromatic 

 condenser from below for transmitted light and dark ground 

 illumination, and also with low and high powers, and even oil- 

 immersion objectives from above. 



For Pond-life work the Wenham binocular is decidedly to be 

 preferred to the monocular microscope. It is less tiring to the 

 eyes to look with both eyes, and without strain, and the 

 stereoscopic image gives a very much better idea of the true 

 shape of the animals, though the images are not quite so sharp 

 as with the monocular tube : but this binocular form can 

 immediately be changed into a monocular for high power work, 

 or whenever desired by pushing the small prism out of the way. 



The binocular is to be used only with the low powers up to 

 the frd inch objective; with higher powers the stereoscopic 

 effect is lost, because the depth of focus, or the plane of distinct 

 vision, is then exceedingly small, and becomes more and more a 

 mere optical section of the object. 



A mechanical stage is hard!}' necessary ; for ordinary work, a 

 well-made sliding stage or bar is preferable, and should be 

 provided. Stage clips, of which opticians are so fond, are 

 abominations, and should be consigned to the dust bin. 



Of illuminating apparatus, the Abbr form of substage 

 condenser, achromatic if possible, is the only one that is really 

 useful for all powers, and that need he considered both for 

 transmitted light and for dark ground illumination. It should 

 be provii^ed with an iris diaphragm and an arm carrying a 

 central stop, and then completely replaces all the older substage 

 apparatus— condenser, spot lens, paralnloid, etc. Tue bull's- 

 eye stand condenser, however, is necessary to render parallel the 

 rays of the lamp flame, but it should be mounted on the lam]), 

 and move about with the lamp, and so as to project an enlarged 

 image of the edge of the flame on to the flat mirror of the 

 microscope for dark ground illumination with low powers. 



All apparatus used in the examination of Pond life — troughs, 

 live boxes, compressors, and pipettes -should always be carefully 

 cleaned and dried immediately after use, and in no case should 

 the water be allowed to evaporate in them. -Much trouble will 

 be saved by the observation of this rule, and the apparatus will 

 always be ready for use. 



" ULTRAMK'ROSCOPICAL" PARTICLES. 

 The meeting of the Ra3al Jficroscopiail Society, held on the 

 17th June last, proved to be the most interesting and successful 

 of the session. It had been anticipated that Lord Raylcigh 

 would have been present to read his paper on " The Theory 

 of Microscopic Vision," but he was unfortunately unable to 

 attend, and the paper was read by Dr. Hobb. The great feature 

 of the evening then beainie a paper by Dr. II. Siedentopf, a 

 member of the scientific st;ift of the Jena Glass Works, dealing 

 with the visibility of ultra-microscopical particles, illustrated 

 by several objects arranged in the manner prescribed by him, 

 and demonstrating the subject in a very wonderful manner. 



