April, 1915. 



KNOWLEDGE. 



115 



on the other hand, days of wintry rigour are experienced, 

 with heavy showers of hail or snow. Hence the old saying 

 " Changeable as an April day." 

 April was a very cold month in the years 1860, 1S79, 

 1SS1, 1884, 1SSS, and 1908 ; and it was' very mild in the 

 years 1844, 1865, 1869, 1874, 1893, and 1894. 



The average mean temperature at Greenwich for April 

 is 47°-3 ; in 1865 it was as high as 52 : -9; while in 1860 it 

 was as low as 43-3. The average maximum temperature 

 is 57-2; the highest mean was 66"-3, in 1865, and the 

 lowest 51°-7, in 1908. The average minimum temperature 

 is 39° -0 ; the highest mean was 42-5, in 1850, and the 

 lowest 34°-2, in 1852. The absolute highest temperature 

 recorded was 81°-5, in 1865, on the 27th, and the absolute 

 lowest 23° -0, in 1847, on the 17th. The average number of 

 da}"s on which the temperature falls to or below the freezing- 

 point is three. 



The average rainfall for the month of April is 1 -61 -in. .' 

 the greatest amount was 4-35-in., in 1829, and the leasl 

 006-in. in 1817. The heaviest fall in one day was 1 50-in.. 

 in 1910, on the 16th. The average number of " rain days " 

 [i.e., on which 01-in. fell) is 12-0 ; the greatest number of 

 days was twenty-one. in 1848, and the least two, in 1852. 



The average amount of bright sunshine at the Kew 

 Observatory, Richmond, is one hundred and fifty-two 

 hours, or about five hours per day. 



The average barometric pressure in London for April 

 is 29-933-in. ; the highest mean was 30-308-in., in 1817, 

 and the lowest mean was 29-590-in., in 1829. 



" April showers bring forth May flowers." 



THE RAINFALL OF 19 T4— Dr. 11 R. Mill, the Director 

 of the British Rainfall Organisation, in his account of the 

 principal features in last year's rainfall, says that the note- 

 worthy features of the monthly rainfall were the prevailing 

 wetness of the four months February, March, November, 

 and December. Of these February was wettest in Ireland, 

 and December in Great Britain. The rainfall of December 

 was exceptional in all parts of the Kingdom, but especially 

 so in the South of England, where it was probably unpre- 

 cedented in this respect. Of the remaining months, January 

 was everywhere dry. and, following an unusually dry De- 

 cember, gave a character to the midwinter season of 1913- 

 14 greatly in contrast with that exhibited in the same period 

 of 1914-15. From April to October the rainfall was in 

 general well below the average, though local thunderstorms 

 in England brought the general fall of July slightly above 

 the average, whilst that of August was above the average in 

 Ireland for a similar reason. The persistent dryness of the 

 seven months, and in particular the small rainfall of ( ►ctober, 

 which was the driest month of the year, was probably as 

 abnormal as was the great rainfall ol December. Taking 

 the British Isles as a whole, it appears that the rainfall for 

 the year was six per cent, above the average, and so 1914 

 must be classed as a moderately wet year. 



AUSTRALIAN WEATHER, 1911 —The following are 

 the results of some of the observations t"i the year 1911 

 at several of the principal towns in Australia as given in 

 The Monthly Weather Report recently issued by the Common- 

 wealth Bureau of Meteorology : — 



Mean Sunshine 



Temperature Rainfall Evaporation. Hours. 



in. in. 



Perth 63-2 ... 23-38 ... 66-34 ... 2961 



Alice Springs 70-0 ... 7 09 ... 97 10 ... — 



Adelaide 62-9 ... 15-99 ... 48-14 ... 2415 



Brisbane 69-3 ... 35-20 ... 4 l >34 ... 2510 



Sydney 63-4 ... 50-24 ... 37-45 ... 1996 



Melbourne 58-6 ... 36-61 ... 38-87 ... 1755 



Hobart 54-6 ... 26-78 ... 31-98 ... 195(1 



MICROSCOPY. 



By J. E. Barnard, F.R.M.S. 



LIGHT-FILTERS.— The wave-length of the light used 

 for illuminating purposes has a very definite influence on 

 the resulting microscopic image. A filter may be selected 

 for one of three reasons : to improve the correction of the 

 optical system — and this applies to both the substage 

 condenser and the objective — to increase or decrease con- 

 trast in the image, or to increase resolving power. It is 

 possible to combine, at least in part, all three qualities 

 n one filter, although it is obvious that to obtain the best 

 result each factor must be dealt with independently. In 

 these days, objectives are so well corrected that the visual 

 image is almost free from any very definite colour aberration 

 but under certain exacting conditions it may become evi- 

 dent. A filter which will cut out the particular colours for 

 which the objective is not corrected can easily be made. 

 It may be assumed that objectives are corrected for that 

 part of the spectrum which has the highest visual luminosity, 

 the region of the green, yellow, and blue-green. It follows 

 that if the colours at the extreme ends of the spectrum 

 are eliminated — the red and violet, for instance — 

 the resulting image will be free from any outstanding 

 colour. There is no better filter for this purpose than that 

 known as Gifford's F-line screen. It is made by dissolving 

 malachite or methyl-green in hot glycerine, the strength 

 of solution depending on the thickness of screen used. In 

 addition, a thin piece of signal-green glass must be immersed 

 in the cell, to cut out the outstanding red which otherwise 

 passes. Alternatively, the cell may be made up with one of 

 its sides of signal-green glass, that is, if the cell is of the type 

 which can be taken apart for cleaning purposes. The only 

 objections to the Gilford screen are that the dyes mentioned 

 are not very stable in solution, and that the luminosity 

 of the screen is not high, as its maximum transmission is 

 in the blue-green. A screen that is quite permanent, and 

 that has higher luminosity, is Zetnow's filter. This is 

 perhaps the most useful of any, either for visual or photo- 

 micrographs purposes. It allows light of wave-lengths 

 between five hundred and seventy and five hundred and 

 fifty micro-millimetres to pass. It is made by dissolving 

 ten grammes of pure dry cupric nitrate and cue gramme of 

 chromic acid in fifty cubic centimetres of distilled water. 

 The thickness of liquid should be one centimetre. If it is 

 not convenient to have so thick a cell, the solutions may. 

 of course, be more concentrated, the relative quantities of 

 each substance being maintained. Another method is to 

 keep read}' saturated solutions of copper sulphate and 

 bichromate of potash, and to mix them as required A 

 precipitate is thrown down, which dissolves in glacial 

 acetic acid. The screen may thus be varied from yellow- 

 green to blue-green to suit the objects under examination. 

 The influence of a suitable screen on resolution is well 

 known, although it is perhaps not so thoroughly realised 

 in practice as it might be. Resolution is dependent on the 

 wave-length of the light used in the medium between the 

 object and the front lens of the objective, and on numerical 

 aperture. The latter factor is a constant, that is. assuming 

 that the full aperture of the objective is utilised The former 

 is a variable, and. in the ca>r • •! white light. i> the 

 mean wave-length of the light emitted by the radiant. 

 Reduction of the mean wave-length therefore, by cutting 

 out the red. for instance, increases possible resolution. 

 By using a screen which effectively cuts out the red and 

 yellow, improvement may be effected The method i- 

 tairlv obvious, but further reference to the matter will 

 be made in this column in succeeding numb 



THE QUEKETT MICROSCOPICAL CLUB.— The five 

 hundred and fifth ordinary, which was also the forty-ninth 

 annual, meeting of the Quekett Microscopical Club was 

 held on Tuesday, February 23rd. at 20, Hanover Square, 

 W., the President, Professor Arthur Dendy. D.Sc, F. R.S., 



