214 



KNOWLEDGE. 



[SEPTEStBER, 1902. 



raov.iblo pointed indicators. Tho following is the method of 

 iisin;,' it : — 



A silvered covor-glass is attjiched to a piece of glass the thick- 

 ne.HS of an or<linarv slip, the centre of the silver being removed 

 for an apoiture of about one mm. The objective is focussed 

 on the centre of the disc, and if the eyepiece then be removed, 

 the whole of that part of the protractor which the objective 

 takes in will be found to be visible at tho back of the objective, 

 and if the pointers be set to the edges of the field the reading 

 cjin be tiiken. As, however, the image seen will be small, it will 

 be neces-ary to enlarge it, and this may be done with a 

 su])plementary lens, jjlaced at the lower end of the draw-tube, 

 replacinfj the eyepiece and focuss-ing the image by means of the 

 draw-tube, the moving parts of the microscope being held the 

 while to prevent the objective from being shifted out of focus 

 on the disc. 



The lens at the lower end of the draw-tube must be a low 

 power one, such, for instance, as the posterior half of a 2-inch 

 objective. The angular reading thus obtained can be converted 

 into numerical aperture by reference to the tables of compara- 

 tive angles and N.A., which are to be found in nearly all text 

 books, or by using a table of sines. 



It should be mentioned that this system is obviously applicable 

 only to dry objectives, but a ready way of approximately 

 ascertaining the aperture of an oil immersion objective is by 

 placing a piece of tissue paper over the front of the objective, 

 holding it towards the light, and looking through the back lens. 

 If the aperture of the objective exceeds I'll X.A., there will be 

 a central annulus of light surrounded by a dark zone ; and the 

 greater the width of the dark zDiie, the more does the numerical 

 ajierture exceed 1(). 



The article, in addition, gives some interesting and valuable 

 information founded on a rational basis regarding the proportion 

 of diameter of stop to be used with the condenser to produce 

 background illumination, with a given objective. 



NOTES AND QUERIES. 



Getil. Warratiil. — The gentleman who acts as consultant re 

 Rotifers replies as follows : — " I am sorry to say I cannot give you 

 much information about the feeding of Rotifers and other forms 

 of pond life, as I have never been very successful myself. I have 

 kept Melioerta for about three mouths, and Stephanoceros for 

 two, but that is about all. I keep my bottles filled with water 

 from a rain tub in my garden, and occasionally put in a drop or 

 two of 'soup' made by pulverizing some Anacharis in water. 

 I do not think an infusion of hay is the slightest use for the 

 purpose." 



-4. ,/. R. — (1) The best condenser for your purpose will be 

 either Baker's achromatic small-sized condenser with a back 

 lens of 22 mm. diameter, or Watson's "Universal." The former 

 is ])robably the more suitable for your work, because it is more 

 useful for iow-power objectives with the top lens removed than 

 the latter. (2j A i-inch objective by any of the leading makers 

 will give you satisfaction. (3) There are several other processes 

 in the preparation of an insect as a micro object. You could 

 probably gain the information you require from any of the 

 books on mounting objects. (4) The 1-inch objective and "B " 

 eyepiece is the most generally useful for examining crystals 

 with the polariscope. 



C. .1/. — I have submitted your drawing to consultants, and, so 

 far as can be ascertained, it is likely to be that of the Common 

 Phantom Larva, Corelhm. phimicornis. I am much obliged for 

 your kind otter, but the delay that necessarily occurs in replying 

 to queries will probably preclude your gathering them. 



F. C. P.— Synapta is a genus of vermiform Echinodermata of 

 the order Apoda. Anchors and plates serve to aid in locomotion, 

 adhesion, and, it is supposed, for impaling food. The species 

 with these plates, &c., are usually of foreign origin. 



Communications and enquiries on Microscopical matters are 

 cordiaUy invited, and should be addressed to M. I. CROSS, 

 KNOWLED(iK Office, 326, High Holhom. W.C. 



NOTES ON COMETS AND METEORS. 

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



Swift's Comkt (1895 It.).— This faint comet was discovered in 

 Pisces on 1895, August 20, and observed for several months. Elliptical 

 elements were computed for it by Berbcrick (period 7'0o years), 



Boss (period 722 years), Schidhof (period 7'19 years), ami others. 

 Scliulhof's period is liable to an uncprtainty of only 15 days The 

 return of the comet is shortly expected, mid the followinf; ephcmeris 

 has been computed by F. K. Scagi-ave, though the conditions are not 

 favourable : — 



DIstiince 

 K.A. Dec. in Millioiui 



II. M. 8. ° ' " of Mites. 



September 2 ... 10 15 55 - 17 6-1 88 115 

 10 ... ]« 31 10 - 18 46 21 116 

 17 ... 16 47 55 - 19 34 56 117 

 24 ... 17 6 1 - 20 18 28 117 

 October 1 ... 17 25 21 - 20 55 10 118 

 8 ... 17 45 51 - 21 23 23 118 

 Thus the comet pursues an easterly course from Scorpio to 

 Sagittarius. 



Heuve Fate. — The death of this eminent French astronomer 

 occurred on July 4th, at the age of 88 years. Born in 1814, Faye's 

 early inclinations led him to the pursuit of astronomy, and on the 

 initiative of Arago he became an assistant at the Paris Observatory. 

 On 1843, November 22, he discovered a small comet nith a fan- 

 shaped tail in the northern region of Orion, and it remained visible 

 imtil 1844, April 10. Dr. Goldscmidt, of Gottingen, and others, com- 

 puted the orbit, and found it an ellipse of short period (7^ years). 

 M. Le A'errier predicted that the comet would return to perihelion on 

 1851, April 3, and the prediction was accurately verified by the event. 

 M. Faye was better known as an astronomical writer than as a 

 telescopic observer, though his name has long been familiar to us by 

 its association with one of the most notable and best known of our 

 short period comets. It last returned to perihelion in March, 1896, 

 and its next apparition will occur in the autumn of 1903. 



Brilliast FrBEBALL. — One of those vivid meteoric apparitions 

 which now and then startle observers who are little prepared fcrsuch 

 spectacles was presented on Sunday night, July 13th, at lOh. 30m. p.m. 

 The firmament seems to have been clear generally, and the descen*. 

 of the fireball was witnessed by a great number of persons. The 

 position of the object was over the Straits of Dov. r, so it was 

 seeu in the S.E. sky, the direction of course varying somewhat 

 according to the station. Numerous descriptions of the phenomenon 

 have been published in the newspapers, but 19 out of every 20 of 

 these afPord no usef id material for computing the meteor's real path 

 in the air. Most of the spectators content themselves with alluding 

 to its sudden and dazzling lustre, to the serpentine trail which 

 lingered for many seconds after the flash, and to the general direction 

 in which it appeared, but give no exact information as to the path 

 traversed. Fortunately, however, a few of those who saw the object 

 were astronomical observers, who recorded its chief features, and 

 amongst these were Mr. W. E. Besley, of Clapham, S.W., Mr. E. 

 Holmes. Hornsey Rise, N., Mr. T. Crumplen, Hungerford Road, X., 

 Mr. E. Rabone, Highgate, N., Mr. E. W. Barlow, Surbiton, Surrey, 

 Mr. W. H. Maw, Outwood, Surrey, Lieut. -Col. Tupman, of Harrow, 

 and the writer at Bristol. It is not essential to give the descriptions 

 in detail, nor can space be afforded for the purpose, but it may be 

 briefly stated that as seen from London the meteor was projected on 

 the sky in the immediate region of the stars forming the constellation 

 Delphinus. Mr. Beelcv, at Clapham, S.W , recorded the path as 

 from 311° + 21° to 3101° -H 1(5J°, while Mr. Rabone, at Highgate, ^■.. 

 put it as 3121° -H 14° to 312J° + 11°. At Bristol the -nTiter registered 

 it as 329 J° -H 12° to 332° -^ 8°. The meteor was seen at such distant 

 places as Liverpool, Shrewsbury, Aberdare, and Moreton Hampstead, 

 Devon. Comparing the best observations the radiant point is indicated 

 at 315° + 31"^ near ^ Cygni, and the meteor fell from 89 to 51 miles 

 from E. of Boulogne, France, to E. of Dungeness, coast of Kent. Path 

 51 miles, and velocity about 26 miles per second. Had the meteor 

 been able to withstand disruption it would have fallen upon the earth 

 near Uckfield, Sussex. Mr. Besley has also determined the real path, 

 and finds that the meteor descended from a height of 86.V to 52^ miles 

 over 11 miles W. of 8t. Omer, France, to 11 miles AV. of Cape Oris 

 Kez. Length 45 miles, velocity 22i miles per second. Radiant 

 316° -I- 30". The fireball had rather a short path, and it seems to 

 have been isolated, for no other meteors belonging to the same radiant 

 were seen by the writer at Bristol, either on Jidy 13th, or on the 

 clear nights preceding and following that date. 



Fireball op July 15th, 9n. 32m.— Though less brilliant than the 

 fireball of July 13tli, this object was conspicuous from its long (light 

 and slow motion. It divided into two near the end of its course. 

 Directed from a westerly radiant in Libra, at about 236° — 12°, it 

 passed over the counties of Essex and Suffolk, descending from 60 to 

 38 miles in height. Its luminous course was about 51 miles long and 

 velocity 17 miles per second. This real path was computed by Hrof. 

 A S. Herschel, who fortunately observed the meteor from near 

 Windsor, and recorded it« flight 'as from 303° +33.;° to 3 VA" + 30J°, 

 and its duration as three seconds. 



