f)2 



KNOWLEDGE 



[April, 1902. 



tliiui water, such us alcohol, as well as fluids of greater 

 ('.cnsity, such as glyceriue, are unsuitable because they set 

 ii|i slrotii; (lift'usion-currcuts l)y osmosis, which cause the 

 animals either to swell or to shrivel up com|iIi'teIy. 



Some species of Kotifers, such as Triarthrn, Polyarthra, 

 I'i'diilioii, MaMigocerca, etc., have an outer surface which 

 is stron<.;ly water-repellent, and when th(!se come in con- 

 tact with the surface iilm of the fluid even for an instant, 

 it is most difficult to submerge them again, and as a rule 

 they are lost or sj)oiled. 



Having then successfully narcotised, killed and fixed 

 the Rotifers fully extended, and finally transferred them 

 into 2^ ])er cent, formalin, the animals may be kept in 

 little bottles, or mounted in the same fluid on micro slides, 

 either in excavated cells or shallow cement cells. 



To mount on a slide, place a drop of the formalin 

 solution in the cell, then transfer the prepared Rotifers 

 into tliis drop with the pipette and examine under the 

 dissecting microscope to see that no particle of foreign 

 matter has been introduced. Then place another drop of 

 the fluid on the slide by the side of the cell and lower the 

 cleaned cover-glass on that drop, then push the cover 

 cautiously and gradually over the cavity. The super- 

 abundant fluid is removed with blotting paper, and the 

 slide closed by tipping damar-gold size cement all round 

 the edge with a fijae brush. 



The permanent closing of these cells has been a matter 

 of very considerable difficulty. As the result of tlie ex- 

 perience gained I recommend to close the cells first with a 

 coat of a varnish consisting of two-thirds damar in benzole 

 and one-third gold size, then two coats of pure shellac 

 dissolved in alcohol, and finally four or five ooats of pure 

 gold size, with an interval always of twenty -four hours for 

 each coat. 



By the method described above I have in the course of 

 the last ten years made a collection of over five hundred 

 slides containing nearly thrfe hundred different species of 

 Rotifers, probably the only collection of the kind iu exis- 

 tence, which is of the greatest use for the identification of 

 species and for the general study of this interesting class. 



Conducted by M. I.Cro§§. 



PoND-i.iiE Coi.LiXTiNO IN APRIL. — All species of Infusoria 

 and Rotifera mentioned as occurring in March are likely to 

 become more abundant in April, which is one of the best 

 months for collecting. The ponds are full of water, whilst 

 they have become approachable, and Daphnias and Cyclops have 

 not yet crowded the Rotifers out, as sometimes occurs later on. 

 Volvtix ijlobator may be looked for together with the little 

 parasitic Rotifer, Prnales parasitica, inside the green spheres. 



Of larger Infusoria, Bursaria liumatcUa, Chuenia Itrm, 

 AinpliHe2itus :/igas andjlagellatus will be found, and, of course, 

 crowds of Ewjlena riridis. 



Of Rotifera, Synchaeia pectinata will be abundant, and 

 Asjilaiichna priodoiita and hrightwelli will have made their 

 appearance in larger lakes and canals ; also Jirachinniis pala, 

 guailratus, and balceri ; Euchlanis triquctra and liyalitia, 

 Triarthra lonr/isela, Diaschi:a semiaperta, lihinops vitiea, 

 I'terudina 2i(itiiM, Jlastiyocerca hicornis, and many others. 



Ti;sTlN(; OiLilcCTlVKS. — Tlie Ahhp Text /'late. — Full directions 

 accompany the test plate, so that the possessor can quickly 

 learn how to use it. It may, however, be stated that it consists 

 of a series of six silvered discs of cover-glass attached to a 

 H" by 1" slip, each cover-glass being of a different and specified 

 gauged thickness, and havinif lines ruled upon it in a deposit of 

 silver on the under-side. These ruled lines are coarse, and can 

 be separated by low-power objectives. 



Information concerning the objectives is gained by the 

 appearance of the edges of each of these lines when illuminated 

 with a condenser of suitable aperture and an eyepiece of high 

 power with a central beam, by oblique illuminations, and by 

 rotating the stage, the correct thickness of cover having first 

 been found by experiment on the test plate. 



The condenser used should yield a solid cone of illumination 

 equal to at least two-thirds of that possessed by the objective, 

 and after setting the tube length of the microscope to that for 

 which the objective is best corrected, the procedure should be 

 as follows : — The ditferent discs should be examined until the 

 one is found in which the lines are seen most distinctly. If 

 under no cover good definition can be obtained, nor by a 

 reasonable alteration of tube length, the objective may safely be 

 rejected as bad. It will probably be found, however, that under 

 one of the covers the lines will appear satisfactorily defined, 

 and it will be possible then to ascertain its corrections for 

 spherical aberration, chromatic correction, centreing, etc. 



Sjifierica! Aberration. — If the objective will bear an eye- 

 piece of high power satisfactorily on the test plate, it is in 

 itself evidence of good correction in this respect. Further 

 evidence may be obtained by placing a small diaphragm beneath 

 the condenser, so that a cone equal to not more than oue- 

 quarter of the numerical aperture of the objective may be 

 yielded, and while looking at the lines, change the position of 

 this diaphragm from central to extremely oblique, the obliquity 

 being in a plane at right angles to the direction of the lines. If 

 the lines remain sharp throughout, the spherical correction is 

 excellent, but if there is a difference of focus, which is stUl 

 apparent on trying other thicknesses of cover-glass and altering 

 the tube length, and it is impossible to overcome a difference of 

 focus for the intermediate position of the diaphragm, the exist- 

 ence of a spherical zone is proved. 



It may be mentioned incidentally that the above method is 

 one of the best for discovering the best tube length and thick- 

 ness of cover-glass for a given lens. 



Chromatic Correction. — This should be tested for in the same 

 manner as the spherical aberration. Under these conditions an 

 apochromatic objective should show practically no colour, or at 

 the very most barely distinguishable traces of tertiary colour. 



Well corrected, or semi-apochromatic objectives should .show 

 narrow bands of pale green (apple-green) on one side, and a 

 peculiar purple (or claret) on the other s^ide of each line, and 

 should show the same colours for all obliquities— the width of 

 the bands only changing — and give good definition with all 

 obliquities. 



Poor objectives will show broad bands of primary colours 

 (yellow and blue) with very obUque light, and the definition 

 will be found to be bad. 



Centreiiiy. — Inaccuracy of centreing is shown by unequal 

 definition of the two edges of the central line : that is, one edge 

 will appear sharp or nearly so, whilst the other will be hazy or 

 seen double, also the coloured edges will be more apparent on 

 one side of the field than on the other. 



There is one point that has to be guarded against, and that is 

 the mistake of supposing that want of sharpness on the margin 

 of the field when the centre is in focus is due to spherical 

 aberration or inaccurate centreing. It is really due to want of 

 flatness of field, which is inherent in the construction of all 

 latter day objectives, and particularly so in those of high power. 



There is, however, a curvature of field due to coma, iu which 

 case only the line or lines about the centre of the field can be 

 focussed sharply, those near the margin remaining indistinct 

 under any circumstances. 



Definition. — A good standard test for definition is to use such 

 an eyepiece with the objective as will give a magnification in 

 diameters equal to one thousand times the numerical aperture 

 of the objective. Supposing therefore a A objective, having 

 an initial magnifying power of 20 diameters, had a numerical 



