190 



KNOWLEDGE 



[August 1, 1899. 



to the following points connected with the display, viz. : — The 

 date and hour of maximum ; the horary rate of the Perseids, and 

 the exact situation of the radiant point on each night of 

 observation. 



In 1898 the maximum occurred on August 11th, and it is 

 certain that for some time in the future the shower will be at 

 its best on a similar date and not on August 10th, as hitherto. 

 The fact that 1900 will not be leap year favours this conclusion. 



The position of the radiant will be as follows ; — 



Date. 

 Aug. 3 

 ., 6 

 „ 9 



36-7° 

 40-2° 

 43-8° 



66-0° 

 56-8'> 



Date. 

 Aug. 12 

 „ 15 

 „ 18 



a 



il-b" + 

 51-3° + 

 55-2° + 



57-5° 

 58-2° 

 58-9° 



The accuracy of these positions should be tested by further 

 observations. 



By John H. Cooke, f.l.s., f.g.s. 



According to Nocht, the success of the nucleus stain of 

 Romanowsky, a mixture of eosin and methylene blue, depends 

 upon the presence of certain impurities in the methylene blue. 

 To obtain the best results the use of polychromic methylene 

 blue is suggested, as the essentials for the formation of the 

 nucleai- stain are more frequently met with in this than in any 

 other. Before using, its alkaline reaction should be neutralized 

 with acetic acid, and the solution should then be mixed with 

 ordinary methylene blue until it is clear and blue. Finally 

 dilute the fluid with a one per cent, aqueous solution until a 

 reddish tinge is apparent near the edges. Macerate the prepara- 

 tions in this for some time, and if too much stain is taken up 

 decolourize with dilute acetic acid. 



The causes of the colouring of leaves in autumn has formed 

 the subject of a series of investigations by Mr. E. Overton. 

 The leaves that turn red he classifies under two heads, those 

 which remain throughout the winter, and those that fall soon 

 after their change of colour. Microscopic examination show 

 that in both cases the palisade cells, and the cells that line the 

 air-chambers of the leaf, are charged with a red cell sap of the 

 nature of glucosides. The cultivation of Hydrocharis moi-sus- 

 raiue, of Utricularia, and many other land plants, in a weak 

 solution of glucose, confirmed his deductions. In each case the 

 leaves assumed a rich reddish-brown tint. 



For general botanical work the most useful killing and fixing 

 agents are solutions containing chromic acid. In the last issue 

 of the Journal of Applied Microscopy, Prof. C. J. Chamberlain, 

 of Chicago University, discusses the results of his experiments 

 with the chromic acid groups, in the course of which he gives 

 some valuable notes on the strengths of the solutions used. 

 For spirogyra, fern prothallia, and similar objects, he suggests 

 a solution made up of chromic acid, two grammes ; acetic acid, 

 one cubic centimetre ; and water, ninety-seven cubic 

 centimetres. If plasmolysis takes place, weaken the 

 chromic, or strengthen the acetic, since the chromic has a 

 tendency to produce contraction, and the acetic to cause 

 swelling. Too large a proportion of acetic acid, however, may 

 cause distortion, and hence it would be better to weaken the 

 chromic acid. 



Referring to the time that should be allowed for the fixing 

 of tissues in chromic solutions, Prof. Chamberlain has found 

 that twenty-four hours should be the minimum even for the 

 most delicate objects. It is well known that zoologists allow 

 fixing agents like Mailer's fluid and Erlicke's fluid to act for 

 weeks before the material is passed on to the next stage, and it is 

 therefore questionable whether the time which is usually allowed 

 by raicroscopists when using chromic acid solutions is not much 

 too short. Sixteen to twenty-four hours is the time usually 

 allowed ; but Prof. Chamberlain's experiments show that the 

 material is better able to withstand subsequent processes if it 

 has been kept in the fixing solutions for two or three days. 

 More rapid penetration, and consequently more immediate 

 killing, can be secured if the reagent is kept at a temperature 

 of from thirty degrees to forty degrees Centigrade. 



Wickersheim's Preserving Fluid is a valuable reagent, but it 



is not commonly used owing to the poor preparations that have 

 been put on the market. Animal and vegetable bodies 

 impregnated with it retain their form, colour, and flexibility in 

 the most perfect manner. The objects to be preserved are 

 placed in the fluid, and left in it for from six to twelve days, 

 after which they are dried in the air. The ligaments remain 

 soft and movable, and the animals or plants remain fit for 

 anatomical dissection and study for long periods. The formula 

 for the fluid is as follows: — Dissolve one hundred grammes alum, 

 twenty-five grammes common salt, twelve grammes saltpetre, 

 sixty grammes potash, ten grammes arsenious acid, in three 

 thousand grammes boiling water. Filter the solution, and when 

 cold add ten litres of the liquid to four litres of glycerine and 

 one litre of methyl alcohol. 



For the preservation of arachnids and myriapods the following 

 mixture is recommended : — Glycerine and Wickersheim's 

 fluid one and a-half ounces each, and distilled water three 

 ounces, the whole to be shaken and thoroughly mixed and added 

 to thirty ounces of ninety-five per cent, alcohol. He considers 

 that alcohol that has been previously used for preserving mites 

 and spiders is preferable to pure alcohol, as the former already 

 contains some of the fats dissolved out of the specimens. This 

 liquid preserves the colouring of the specimens, and keeps 

 them flexible. 



For five months Dr. Marsson concentrated his attention on 

 the study of the variations of the animal and plant life of the 

 plankton of the Leipsig ponds, one result of which has been the 

 discovery of many new and interesting — though anomalous — 

 facts. Two ponds, separated only by a road, never contained 

 the same forms, ^'olvox aureus was found in abundance in the 

 pond on the south side, but not a single specimen could be 

 found in that on the north. Syiiura uvella was found in the 

 one, in September, in great quantities, and none at all in the 

 other. Both ponds afforded similar conditions of depth, 

 character of soil, light, and plant growth, and swans and other 

 water birds frequented both. On the 20th May, Tintimdium 

 fluviatiJe made its first appearance in a pond, and on the 26th it 

 formed the largest constituent of the plankton, after which it 

 disappeared and did not return. In April, Codanella lactistris 

 appeared in this same pond, then it disappeared entirely, and 

 was firdt found in other ponds in August. 



To many microscopists the terms " one quarter inch," or 

 "one half inch," as applied to their objectives, convey the idea 

 that, when in focus, the object is at a distance of a quarter 

 inch, or a half inch, from the front lens. They confound the 

 equivalent focal length of the objective with the working 

 distance. As a matter of fact, the latter is always considerably 

 less than the former. The determination of the working 

 distance of an objective is a point of considerable importance, 

 and therefore all microscopists should make themselves familiar 

 with the method of calculating it. The following simple device 

 will be found useful for estimating the working distance of 

 objectives that are not higher than one-twelfth inch. Make a 

 long thin wooden wedge, ten centimetres in length along the 

 base, and twenty millimetres in perpendicular height. Focus a 

 diatom on a glass slip without a glass cover, and then carefully 

 push the wedge along the glass slip until it touches the objective. 

 The thickness of the wedge at the point of contact will 

 represent the working distance of the objective. 



Prof. Leroy gives, in the Journal of Applied Microscopy, the 

 results of his experience in the use of picro-carmine as a 

 counter-stain for bacteria in tissues. As a rule this reagent is 

 somewhat uncertain in its effects, and it is therefore suggested 

 that, to obviate risk of failure, the tissues should be first treated 

 with logwood picro-carmine, and finally stained by Gram's 

 met'nod of bacterial staining. As a counter-stain, alum carmine 

 alone gives only a nuclear stain and leaves the cytoplasm 

 practically untouched. Better results can be obtained by first 

 staining in alum carmine or borax carmine, then carrying the 

 section through the regular Gram process, and lastly leaving 

 the section for half a minute in a solution of sodium sulp.- 

 indigotate O'l gramme, and carbolic acid, five per cent. aq. sol. 

 one hundred cubic centimetres, after which follow on with 

 alcohol, creasote, and balsam. By this method the nuclei will 

 stain red, the cell bodies apple green, and the bacteria purple 

 (if gentian violet be used in the Gram's solution). 



