118 



KNOWLEDGE. 



[May 1, 1900. 



screen as thus prepared with a clear centre, servos both for 

 the general focussing and the finest focussing, and avoids the 

 danger of using the double screen. 



For the photography of opaque substances, such as metals, &c., 

 a metal microscope, such as that which is made by Reichert, 

 of Vienna, is necessary. The microscope must be fixed in 

 an upright position, and reflected light used. The source of 

 light should be at a distance of one to one and a half metres 

 from the apparatus, and must be on the same level as the 

 reflecting lens at the side of the vertic:il illuminator on the tube 

 of the microscope. The specimen should be everywhere equally 

 illuminated and then focussed. Eosin plates and the use of a 

 yellow screen are to be recommended for. this work. 



The selection of plates and .screens for photo-micrography is 

 a subject upon which we propose to say more in future issues. 

 The following points are of practical importance, and should 

 receive careful attention : — For stained preparations ortho- 

 chromatic plates give the best results, but it is of advantage to 

 place a screen, complementary in colour to the stain used, 

 between the source of light and the microscope. Generally 

 speaking, a light filter of picric acid should be used for 

 specimens stained dark red, or violet ; for light red staius, a 

 greenish-yellow one ; and for preparations stained with methylene 

 blue, a dark orange-jellow filter is recommended. 



[•-1/^ C(jiniaunicaUo)is in reference to this Column xhoiiltl be 

 adilresaed to Mr. J. H. Cooke at the Office of Kxowledgf.,] 



NOTES ON COMETS AND METEORS. 



By W. F. Denning, f.r.a.s. 



Thr Di.scovery of Comkts.— In the century from 1700 to 1799 

 inclusive 64 comets were discovered and observed sufficiently well 

 for their orbits to be computed. The average rate of discovery was 

 therefore 0.64 annually or about two comets every three years. A 

 vast increase in the number of these discoveries is shown by the 

 figures for the century from 1800 to 1899. 315 comets were found, 

 including redetectiims of periodical comets, so that the annual 

 average was 3.15. Comparing the first with the last half of the 

 century the numbers were — 



Comets AnniKil 



found. .averiitre. 



1800 to 1849 91 1.8 



1850 to 1899 224 4.5 



There were comparatively few comets found before 1840, but in 

 that year and the few ensuing ones a marked rise occurred, and 

 observers have been veiy successful in this field ever since. The 

 numbers of comets discovered in the various months of the year 

 during the period 1800 1899 were as follows : — 



January 17 July 35 



February 19 August ... 40 



March 24 September ... 25 



April 27 October ... 26 



May 20 November ... 51 



June 28 December ... 33 



Of the spring months March and April show the best returns. In 

 the summer there is a ra])id rise from June to August, the latter 

 being apparently the best month of the year for sighting new 

 comets. In the autumn, November has a good record, notwith 

 standing the unfavourable weather often prevailing at this period. 



Gi.iCOBiNi's Comet. — During the coming summer conietary ob- 

 servers will have an interesting object for study though it will be 

 bv no means bright. It wiU, however, occupy an extremely favour- 

 able position in the sky during the four months June to August 

 inclusive. At the end of May the comet will become visible in the 

 morning sky, being placed in the N.E. extremity of Pisces and 

 about 7 degrees south of Beta Andromedse. Thence it moves to 

 the north-west, and early in July will be found in the left hand of 

 Andromeda, near Nu, "C^hi, Psi, and Lambda in that constel- 

 lation. It afterwards traverses I>acerta and Cygnus, passing near 

 Al]iha, and at the end of July will be about 2 degrees south of DeU.>. 

 Passing then through Lyra it enters the eastern borders of Hercules, 

 and will be near Alpha Ophiuchi at the middle of September. But 

 its great increase of distance will now have rendered it faint, and 

 its motion south will cany it out of sight altogether. Our last 

 number contained an outline ephemeris by Berberich. and it is 

 intended to give its path with more det.nil so that the object may 

 be attentively followed during ensuing months. 



Stationary Radiation of Mf.tf.ors. — Tins may be s,i,id to form 

 one of the moot points of astronomy. It is not a discordance 

 between rival observers, but a difficulty of explaining observed 

 facts on apiuoved mathematical theories" In 1878, when fixed or 



recurring radiants were brought prominently into notice, it was 

 asserted that they must be due to successive but different showers 

 accidentally grouped in such a manner as to render their apparent 

 directions almost identical. But ob ervation showed that this idea did 

 not satisfactorily account for the facts because the points of diver- 

 gence remained constant (allowing for small and unavoidable errors 

 of observation), and that there were no such dift'erences as a mere 

 chance grouping of streams must certainly occasion. Wherever a 

 radiant is placed relatively to a neighbouring star there it remains 

 during the whole period of its visible activity, and this often covers 

 several months. Of late several able mathematicians have attacked 

 the matter, and valuable papers have appeared from Profs. A. S. 

 Herscliel. H. H. Turner, and G. von Niessl, Dr. Bredikhine, ;ind 

 M. 0. Callandreau. Prof. Turner has offered a very ingenious 

 explan.ation of stationary radi,ation (" Monthly Notices," Jan., 1899), 

 on the basis of planetary perturljaticm operating through vast inter- 

 vals of time u])on meteoric ]}articles, and the observed peculiarity 

 seems in a fair way of being understood. More observations would 

 be very valuable. Very few astrftnomers have ever practically in- 

 vestigated the matter, for the reason that it requires a vast number 

 of very accurate materials before any satisfactoiy tests can be 

 applied as to the stationary aspect of the minor showers generally. 

 Instead of examining the whole question it would be better to select 

 a test case, say that supplied by the Orionids of October. If this 

 notable autunm shower were attentively watched between October 

 12 and 25 and the radiant independently determined on each nlulit 

 it would be easily seen whether the radiant were stationary. And 

 if one well known shower is found perfectly stationary during a 

 fortnight there can be no reason why other showers may not present 

 a similar aspect during even longer interv.als. 



May Meteors. — The spring season may be described as one of 

 meteoric scarcity. The Aquarids, sometimes visible in the morning 

 twilight, and possibly associated with Halley's comet, furnish, how- 

 ever, an interesting system which well deserves looking for. It 

 furnishes fine long meteors with streaks during the first week in 

 May, but they are only perceptible just before daylight as the 

 radiant is below the horizon in the early part of the night. 



FiRKBALL, March 28th, 8h. 31m. — A barge meteor giving two 

 outbursts like vivid lightning flashes was seen by Mr. tj. T. Davis 

 at Reading. He says it ]iassed from near Cor Caroli towards 

 Denebola, but descrilied a curve ending near E)isilon Virgiuis. It 

 exploded twice, finally breaking up into sparks. The meteor lit the 

 place up. Mr. T. H. Astbury, of Wallingford, writes that he was 

 looking north at the time, and only saw the brilliant flashes of 

 light which the meteor occasioned. From inquiry afterwards he 

 found that tlic meteor's apparent path was from about 188" + 18° to 

 193—6", aud that it was very much brighter than the planet 

 Venus. 



THE FACE OF THE SKY FOR MAY. 



By A. Fowler, f.r.a s. 



The Sun. — On tlie 1st the sun rises at 4.33 and sets at 

 7.21 ; on the 31st he rises at 3.51 and sets at 8.3. One 

 of the most striking astronomical events of the year will 

 be the eclipse of the sun on the 28th. Full particulars as 

 to the jiarts of the earth's surface from which the total 

 phase will be visible are given by Mr. Maunder in the 

 March number of Knowledge. Over the British Islands 

 a large partial eclipse will be visible, the data given in the 

 Nautical Almanac being as follows : — 



All the times above are expressed in Greenwich Mean 

 Time, and are p.m. ; the position angles of the contacts are 

 for direct image. 



Three phases of the eclipse at Greenwich are illustrated 

 ill the accompanying diagram, which is constructed with 

 the vertex at the top in each case. 



