166 



KNOWLEDGE 



[July 1, 1899. 



edge of the cover-glass, by means of the turn-table, and after- 

 wards, when set, protecting this in its turn with a ring of finish- 

 ing cement. A very beautiful and instructive preparation may 

 be made by heating xylene balsam on a slide until the xylene 

 has almost evaporated, and then adding a few crystals of 

 sulphonal. The preparation should be gently warmed until 

 the sulphonal melts and mixes with the balsam. The cover- 

 glass is then put on. If perfect crystals are obtained this mount 

 will show, with the aid of the polariscope, not only the most 

 gorgeous colourings, but also perfect examples of the black 

 cross. 



An ocular pointer is a useful accessory for class demonstra- 

 tions when it is desired to indicate any particular structure or 

 object in the field of view. To make one, cut out a circle of 

 cardboard of a size sufficient to fit easily in the ocular tube. 

 Out of the centre of the circle punch a second circle, having a 

 diameter slightly larger than that of the ocular diaphragm. 

 Blacken this cardboard ring, and, with a little gum, fasten an 

 eye-lash to edge of the smaller circle so that the ciliuin shall 

 project haifway across the opening. Remove the eye-lens of 

 the ocular, and drop the cardboard ring into the tube so that it 

 rests on the diaphragm. The c/h'iiin is now at the level of the 

 real image, and the specimen on the stage of the microscope 

 can be so placed that the pointer will indicate exactly any par- 

 ticular portion to which it may be desired to draw attention. 



Alcohol is used by many microscopists both for killing their 

 specimens and for preparing them for the laboratory and 

 museum. In the case of marine animals the use of alcohol in 

 inexperienced hands is often attended with unsatisfactory 

 results, and this for several reasons. On thick-walled animals, 

 particularly those provided with chitinous envelopes, this 

 reagent acts prejudicially on the internal organs, while in the 

 case of the smaller Crustacea it gives rise to precipitates in the 

 body fluids which frequently prevent a satisfactory dissection 

 of the parts being made. It has also been found in practice 

 that alcohol tends to fix the salts of the sea-water contained in 

 the organism, and thus, by forming a crust, prevents both the 

 hardening and the staining fluids from penetrating 'the tissues. 

 With proper precautions there are, however, few reagents which 

 give better results. The method employed in the Marine 

 Biological Laboratory at Naples is worth the attention of those 

 whose efforts with alcohol have not always been so successful 

 as they may have desired them to be. To kill, let us say, an 

 annelid, the animal is transferred to a beaker containing sea- 

 water. A few drops of alcohol are added, and this is repeated 

 at short intervals until the animal expires. This tentative 

 method of killing neither causes contraction nor distortion of 

 the parts. The animal dies slowly, and, when dead, is so supple 

 that it may be readily arranged in any desired position. The 

 main point is to avoid the mistake, usually made, of killing in 

 strong grades of spirit. After death the specimen may be 

 passed through the various grades of alcohol in the usual way. 



The popular theory regarding " honeydew " is that it is an 

 excretion from aphides. Mr. H. W. Brice has recently made a 

 microscopical examination of the substance, and has arrived at 

 the conclusion that the popular theory is fallacious. Under the 

 microscope some thicklj'-coated leaves of the lime and sycamore 

 revealed not more than three or four insects per leaf. These 

 insects were removed, and an hour afterwards the beads of 

 honeydew were found to be more numerous and larger than 

 when the leaves were first gathered. He concludes, therefore, 

 that the leaves of some trees, under favourable climatic con- 

 ditions, become surcharged with saccharine matter, and, the 

 cells bursting, a copious exudation of " honeydew " takes place. 

 The fact that " dew " gathered from the sycamore and the oak 

 is much darker than that from the lime is held to prove that it 

 partakes of the nature of the tree and not of the insect. 

 ♦ 



NOTES ON COMETS AND METEORS. 



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



Swift's Comet. — In June this comet travelled in a direction 

 which carried it rapidly away from the earth, and in July it 

 will only be visible in powerful instruments. At the opening 

 of the latter month the comet will be situated a few degrees 

 S.E. of Arcturus. The following ephemeris is by Dr. A. 

 Stichtenoth {Ast. Nach., 3574) :— 



R,A. Distance in Bright- [ 



Date. li. m. s. Dec. Millions of Miles, ness. 



July 2 ... 14 20 3« + 19° 39-3' 117 0-13 



„ 6 .,, 14 ir> 54 + 17° 16-5' 129 0-11 



„ 10 ... 14 14 20 -I- 15" 130' 140 0-08 



„ 14 ... 14 12 43 -f 13° 24-'.i' 152 0-06 



The comet has shown some extraordinary fluctuations in 

 brightness. On June 4th and 5th it was estimated one magni- 

 tude brightei than on June 3rd. The sudden increase was 

 noted by Mr. T. H. Astbury, of Wallingford, and it is also 

 referred to in Ast. Nach., No. 3574, by Hartwig Schorr, 

 Holetschek and Pokrowski. 



Tempei.'s Comet (1873 II.) — This object is now favourably 

 visible and rapidly increasing in apparent brightness. It is, 

 however, moving to the S.S.E., and will be somewhat low in 

 the sky as observed from our latitude. Ephemeris by L. 

 Schulhof :— 



R.A. Distance in Bright. 



Date. h. m. s. Dec. Millions of Miles, ness. 



July 3 ... 20 18 43-10° 1' 5" 40 2-77 



„ 7 ... 2t) 23 45 - 11° 38' 27" 38 3-01 



„ 11 ... 20 28 41 - 13° 27' 12" 37 3-23 



„ 15 ... 20 .33 £3 - 15° 25 '46" 36 3-42 



„ 31 ... 20 52 41-24° 7' 36" 35 3-67 



Aug. 4 ... 20 57 26 - 26° 14' 6" 35 3-.59 



The comet will, therefore, be placed in the northerly region of 



Capricornus early in July. 



Holmes's Comet. — This interesting body ought to come 

 under observation in July if serious changes have not affected 

 its appearance or orbit since its apparition of 1892. In Ast. 

 Nach., 3553, Zwiers gives the following sweeping ephemeris : — 

 Date. ^ ^^' g Dec. Brightness. 



July 3 ... l"5l"4i + 24° 7' 0-0374 



„ 7 ... 1 .57 53 2,5° 17' 0-0382 



„ 11 ... 2 3 59 26° 26' 0-0.390 



„ 15 ... 2 9 56 27° 34' 0-0398 



„ 31 ... 2 32 10 -t- 32° 2' 0-0435 



The brightness of the comet in 1892, November 8th, is 

 adopted as 0-0747. On July 9th its position will be three 

 degrees north of the bright star a Arietis, and its motion will 

 carry it slowly to the north-east. 



Since the above was written, information has come to hand 

 that Holmes's comet was re-discovered on the morning of June 

 11th by Perrine, at the Lick Observatory, and that its position 

 differed very little from that given in the ephemeris by Zwiers. 



Tdttle's Comet. — This object is now invisible to observers 

 in the northern hemisphere. During July it will travel rapidly 

 to the south-east, and on the last day of the month will be in 

 R.A. lOh. 10m. 14s., Dec. 30° 54' south. 



FirehaU of IS^djAprilith. — InournotesforMayreference was 

 made to this object as observed at Birmingham and St. Anne's- 

 on-Sea. It was also seen at Chester, and though the several 

 observations are not in good agreement it is certain that the 

 meteor was a brilliant Virginid, the radiant being at about 

 202° — 10°. The length of its real path was about one hundred 

 and sixty miles from a height of sixty miles above a point ten 

 miles west of Whitehaven to fifty-two miles above the north 

 coast of Ireland. 



Firehall of 1899, June 2>id. — In the strong twilight prevailing 

 at about 8h. 47m. on June 2nd, a very fine meteor was observed 

 from several places in the West of England. Few stars were 

 visible at the time, so that the path of the object would not be 

 exactly determined. Mr. Thomas Harries, of Llanelly, describes 

 the meteor as rivalling Venus, and says its path was directed 

 from Leo towards due west, extinguishing there at a point about 

 thirty degrees above the horizon. The duration of flight was 

 four seconds. At Bridgwater the object was seen by two 

 persons, from whose description immediately aftei-wards Mr. 

 Corder placed the beginning near /3 Leonis and the end close to 

 the N.W. horizon. Its brightness was estimated as four times 

 that of Jupiter. At Weston-super-Mare the meteor was seen 

 as " a clear electric ball travelling quite slowly in a direction 

 from east to west, and descending gradually until it seemed to 

 go out." It was visible for two or three seconds, and appeared 

 not very high in the heavens. It was also seen at Bristol and 

 some other stations, but the descriptions are not very exact as 

 regards details. It is certain, however, from a comparison of 



