20 



KNOWLEDGE 



[January, 1903. 



The Milk Vetches (Astragalus) are three in number, and 

 exhibit a great diversity of liabitat, the first (A . gliici/pliiflJos) 

 being a large meadow plant, the next (A. Iiyjxiij/ottis) a 

 small chalk species, while the third (A. alpimiK,, and 

 Oxytropis campestris, which comes next on our list, are the 

 only British species of the Order which affect an alpine 

 habitat, both growing very sparingly on the Scotch 

 mountains at an altitude of 2000 feet or more. 0. uralensis, 

 our only other representative of the latter genus, is a 

 small Scottish hill plant, which ascends nearly as high as 

 its congener. The genus Orniihopus, Bird's-foot, has 

 two species, pretty little sand or gravel plants with pinnate 

 leaves. 0. perptlsiHus has a wide distribution, while 

 0. ehradeatus grows on the Scilly Islands. The Horse- 

 shoe Vetch (Hippocrepis comosa) is a similar looking plant 

 with curious wavy pods, wide-spread in England. The 

 Sainfoin (Onobnjchis sativa) is, like the last, the only 

 British member of its genus. It is a handsome herb, a 

 couple of feet in height, with pinnate leaves and racemes 

 of rose-red flowers, in our islands confined to England, 

 where it is not certainly native. Lastly we have the two 

 large genera, the Vetches (Vicia) and Vetchlings (Lathyrus), 

 which have been frequently referred to above. The 

 climbing sj^ecies generally grow, the lower among grass, 

 the taller among shrubs, where they may attain in the 

 season's growth a height of six or eight teet. The species 

 devoid of tendrils grow in bushy form, or straggle upwards 

 supported by neighbouring vegetation, or lie prostrate on 

 the ground. The climbing species are the most picturesque 

 of all our Leguminosas. With their often winged ribbon- 

 like stems, their graceful tendril-tipped leaves, and 

 handsome cl usters of blossom, they form delightful objects, 

 and when we remember the beautiful and complicated 

 structure of the flower, and the marvellous sensitiveness 

 of the tendrils, we see and recognize in them some of 

 the most highly developed forms to be met with in 

 our whole native vegetation. 



Coiiducttd by M. I. Cross. 



PHOTOGRAPHY OF OPAQUE OBJEOT.S. 

 By Frederick Koad Clark. 



The method of procedure for this branch of Photo-micro 

 grapby, in so far as the microscope is concerned, is practically 

 the same as that for the photography of transparent objects. 

 The difference Hes chiefly in the mode of illumination, as well 

 as in certain precautions to be observed in the mounting of the 

 specimen, and in the purely photographic portion of the work. 



At the commencement it is necessary that the object should 

 appear in contrast to its background, the latter being selected 

 with a view to this. A dark subject should have a light back- 

 ground, and vice versd. For white objects, such as some eggs of 

 insects, Foraminifera, etc., a disc of asphalt varnish placed on 

 the glass .slip with the turntable will be found suitable, the 

 object being fixed thereon with a tiny spot of thin gum 

 tragacanth. For darker subjects the under surface of a thick 

 glass slip may be painted with white cement or enamel, the 

 thickness of the glass minimising the effect of shadows cast by 

 the ilhiminant. Should this not prove satisfactory a disc of 

 white cardboard may be placed over the substage condenser a 

 little distance behind the object, which in this case is mounted 



on a clear glass slip. Fair results may be obtained with speci- 

 mens permanently mounted under a cover glass, but the best 

 are made with those that have been freshly mounted without a 

 cover. 



Whenever possible objects should be photographed on their 

 natural support. For example — the egga of insects in xitd on 

 the larval food-plant, leaf, bud, calyx, etc., the educational value 

 of the result being thus euhauced. This, of cour.se, is not 

 always possible, for frequently, as in the case of insects' eggs, 

 the objects lie in different planes. When two or more objects 

 in a group are photographed it is essential that they lie in a 

 plane exactly at right angles to the axis of the microscope 

 tube, or trouble will result owing to difficulties in focussing. 

 The following plan may be recommended : fix the objects on a 

 cork cut perfectly flat on the upper surface, having its sides 

 tapered off so as to fit the opening in the stage, it may then be 

 readily adjusted in any position. If the object is mounted on 

 a glass slip this may also be attached to the cork. 



Dark ground illumination is effected in several ways. One 

 method is by using the central spot diaphragms, the smallest of 

 these which will give the required dark ground being inserted 

 in the substage condenser. The exposure of the plate will 

 necessarily require to be long. Another method sometimes 

 employed is the reflector known as the " Lieberkuhn." It, 

 however, has disadvantages, as a separate one is required for 

 each objective, and the mount must be specially prepared with 

 a circular black spot on the under side of the glass slip. The 

 illumination also is not satisfactory, being too "general," and 

 delicate structure is flooded with light when contrast is required. 

 Probably the best method is in the use of the side silver reflector, 

 the rays from a good paraffin lamp being condensed on its surface 

 by means of the bull's-eye condenser. Excellent results have 

 been obtained simply by using the bull's eye placed obliquely 

 towards the object, and as closely as possible. These methods 

 are, of course, only applicable to low-power objectives, the short 

 focal distance of the higher powers not admitting of their use. 

 When photographing with the higher powers up to one-twelfth 

 inch the " vertical illuminator " must be employed. This i» a 

 piece of apparatus fitted into the microscope tube above the 

 objective, the hght being conducted thereto through a .small 

 aperture in the side of the tube. When this is used the object 

 must be either uncovered or in absolute contact with the cover 

 glass. We will not notice such devices as the spot lens, ])ara- 

 boloid, etc., for obtaining dark ground illumination, they having 

 been superseded by the methods just described. 

 ( To be continued.) 



PoND-LiPE Collecting in Janqary. — January is the most 

 severe month of the year, and lakes and ponds are often frozen 

 over, or difficult to approach. Microscopic Pond-life, though 

 less abundant than in the spring and autumn, is nevertheless 

 nearly always present, even under the ice many inches thick. 

 All the following species of Rotifers have been taken in 

 January in and near London, but, no doubt, a great many more 

 could be found by .systematic search : AsjilanchiKi hrinhtin'Ui 

 and priodonia, Aiturma aculeata and cochlearL«,iinichioiius paid 

 and angularis, Notholca scaplui, Eivchlanis dejiexa and liyalina. 

 Rotifer macrurus and vulgaris, Polijarthra platyptera, Syncliwta 

 pectinata, tremula, and oblowja, Conochilux unicornis, C'oelopus 

 porcellus, Diaschiza lacinulata and ventr/pes, Proalea decipiens 

 and petromyzon, Diylena forcipala, Dinucliaris pocilhim, Mono- 

 styla cornuta, Colitriis caudatus, Melicerta ringens, Limnias 

 ceratophylli, Oecistes crystalliniis, Floscularia cornuta, and 

 Stephauoceros Eirlihonii. Jjiaptomus and Cyclops and their 

 larvas are abundant, whilst SVaterfleas are almost absent. 

 Aquatic vegetation having died down, the fixed forms of 

 Rotifers and Infusoria should be looked for on the rootlets 

 of trees growing near the edge of the water. Floscules and 

 Melicerta were once found covering such rootlets very thickly. 

 January seems to be the time when the males of Stepltanoceros 

 and other tube dwellers are often found, and their presence is 

 usually betrayed by the thick-shelled, fertilized, resting eggs in 

 some of the tubes, and numerous smaller male eggs in others. 



The Microscopical Work of 1902 has been steadily pro- 

 gressive; there have been no great changes or discoveries, 

 although many additions to and imjuovements in the detail of 

 instruments have occurred and been duly recorded. 



A feature that stands out distinctively is the increased 



