190 



KNOWLEDGE. 



FAdgust, 1903. 



It has of course long been known that whilst there is a fairly 

 well-defined limit of resolving jiower — in other words, a inhunnim 

 distance apart beyond which two or more particles or elements 

 of structure cannot be seen apart — there is no such limit for the 

 size of the separate particles themselves, at any rate not when 

 they are placed in a strong light against a dark background : 

 for under these conditions the smallness of the reflecting surface 

 of the particles can be compensated for by the brightness of the 

 illunaination, in much the same way as minute particles of 

 floating dust become visible in the path of a ray of sunlight 

 through an otherwise shaded room. 



The interesting and novel parts of Dr. Siedentopf's paper 

 were those dealing with the methods adapted for carrying this 

 system of rendering minute particles visible, to its utmost limit, 

 and those which resulted in fairly reliable estimates of the 

 actual size of the particles. 



The usual dark-ground illumination is not very efficient, for 

 whilst it is easy enough to find a stop which will not allow a 

 single ray of regularly refracted direct light to pass through the 

 microscope, it is impossible to prevent a quantity of scattered 

 light produced by reflection from the mounts of condenser and 

 objective, and by double reflections at the numerous lens sur- 

 faces, from reaching the eye and rendering it less sensitive to 

 feeble impressions. 



This difficulty Dr. Siedentopf overcame in a most ingenious 

 manner by taking advantage of total reflection. He arranged 

 his condenser (a dry one) at right angles to the optical axis of 

 the microscope, so that its concentrated light entered the slide 

 containing or forming the object, laterally. It will at once be 

 seen that none of this light can emerge into air through the 

 upper surface of the slide, as this would demand a refractive 

 index of 1'414, — lower even than that of fluorite. The light 

 diffused by small particles is therefore the only light that reaches 

 the eye, and the latter wiU therefore be sensitive to its utmost 

 limit. 



But it is further necessary to limit the illumination to particles 

 within the limits of depth of focus, as otherwise the light 

 scattered by particles not in focus would form a luminous back- 

 ground ; this object Dr. Siedentopf attained by focussing the 

 source of light — generally the electric arc — on an adjustable 

 spectroscope-slit, then focussing the slit in his object. 



By the use of this refined method. Dr. Siedentopf was able to 

 prove by direct observation that the beautiful red glass, known 

 as gold-ruby glass, contains small detached particles of gold of 

 extremely small size. By computing the cubic space under 

 observation, counting the particles within that space, and 

 ascertaining by analysis the percentage of gold in the glass under 

 examination, it was found that the particles were equal in weight 

 to tiny cubes of gold having sides equal to from one-sixth to 

 five one-millionths of an inch. Though almost inconceivably 

 minute, the particles are still composed of a great number (at 

 least 10,0011, according to Dr. Siedentopf) of molecules. These 

 latter must, therefore, still be considered as invisible. 



Very minute particles of other origin were also demonstrated, 

 for instance the flagellse of certain bacteria, and we may confi- 

 dently look forward to important additions to our knowledge 

 resulting from an intelligent application of the methods of 

 illumination and of observation devised by Dr. Siedentopf. 



One interesting question was not, as far as we are aware, gone 

 into by Dr. Siedentopf, viz., why the gold-ruby glass should 

 transmit chiefly red light ; this seems rather puzzling, for thin 

 layers of metallic gold transmit green light. Could the colour 

 arise in a similar manner to the blue of the sky V 



Rudiments of Palpi in Diptera. — It is not generally 

 known that the palpi present on the proboscis of our common 

 house flies, Musca domestica, and Homalomyia canicidar/s, are 

 labial, that is to say, attached to the lower lip. The exceedingly 

 minute remains of the maxillary palpi can be made out on the 

 anterior ends of the levers that work the lancet. These parts are 

 embedded in the proboscis, but if a preparation is mounted under 

 pressure, after having had the soft parts cleared away by caustic 

 potash, the levers can be seen through the transparent membrane, 

 and a good " sixth," with careful substage illumination, will 

 show some hairs on the tubercles at the anterior ends. These 

 parts are the stipites and cardines of the inner jaws (maxills.) 

 A series of palpi can be seen in different species, from a well- 



developed hairy organ, protruded through the membrane, to a 

 small embedded bulb, which has lost the few hairs we see in 

 J/, domestica. This, regarded from the evolutionary point of 

 view, is exceedingly interesting. 



Communications and enquiries on }ficroscoj>icaI matters are 

 cordially invited, and should be addressed to M. I. Cross, 

 Knowledge Office, 326, High Holborn, W.C. 



NOTES ON COMETS AND METEORS. 



By W. P. Denning, f.e.a.s. 



New Comet. — A pretty bright comet, with nucleus and tail, was 

 discovered by Borrellv, at. Marseilles, just before midnight on June 21, 

 in R.A. 211i. o2m. 528., Dee. S. 8^ 10'. The direction of motion was 

 towards N.N.W. On June 30, at Oh. 30m. a.m., the writer, at Bristol, 

 obserred the comet, and found it distinctly viBible to the naked eye. 

 It had a bright nucleus, and a tail of about k degree flowing to 

 the S.S.W. The comet will be in perihelion at the end of August, 

 and during that month will move from Ursa Major, through Leo 

 Minor into Leo. 



Alphonse Borrellv, the fortunate discoverer of this object, achieved 

 distinction as a comet finder a generation ago. He sighted his firet 

 comet on 1871, October 12, and found five others during the six 

 ensuing years. With a single exception, all the comets seen during 

 the ten years, from 1SG8 to 1S77 inclusive, were discovered by one or 

 other of the famous comet seekers, Borrellv, Coggia, Xempel and 

 Winnecke. 



Bbooks's Pebiodical Comet (1889 V.-1896 VI.).— This comet 

 will reach its nearest point to the earth on about August 17, though 

 it will not pass through perihelion until the second week in December. 

 The comet is rather unfavourably situated for northern observers, as 

 it is 27 degrees south of the equator, but it ought to be pretty well 

 seen from stations having a very open, unobstructed view southwards. 

 It is moving very slowly westwards, its place at the end of August 

 being only 5 degrees west from that occupied at the opening of tlie 

 month. The following is an ephemeris for Berlin midnight : — 

 E.A. Declination. 



1903. 

 August 1 1 



,. 19 



n 



26 59 



27 3 

 27 6 

 27 5 

 27 1 

 26 53 



S 20 

 21 5 7 . 

 21 2 

 ,, 23 ... 20 59 4 

 27 ... 20 56 25 

 „ 31 ... 20 54 7 



During this period the comet's distance from the earth wiU be about 

 110 millions of miles. 



DOPBLY-OBSEEVED Meteoks. — The real paths of an interesting 

 pair of bright meteors recorded at the April epoch have been computed 

 by Mr. W. E. Besley and by the writer, and the results, placed side 

 bv side for comparison, are as under : — 



April 23, lOh. 2.3m. AprU 22, 12h. 32iin. 



W. E. B. W. F. D. W. E. B. W. F. D. 



Height at ") ,,-,, ,-_ ^- -o 



° ^ ... 42.',m. 4om. ... lom. 78m. 



appearance ) 



Height at ^ 26m. 26m. ... 41m. 43m. 



disappearance ) 

 Length of path ... 17m. 20m. ... 44m. 49m. 



Velocity per second... ojm. Cim. ... 35m. 39m. 



Radiantpoint ... 15l'>-(-47i° 150"-h49» ... 270' + 3i<' 269<' + 32'> 



Name of meteor ... Ursid. .,, Lyrid. 



C. L. Brook, Meltham....A.S.Herschel,Slough. 

 A. King, Leicester. ...A. King, Leicester. 



There was also a fine Lyrid observed on April 22, lOh. 3Sm., by 

 Prof. Herschel at Slougli, and llr. T. H. Astbury at Wallingford. 

 The heights were from 70 to 56 miles over Northamptonshire. 



FiBEBAU. — Mr. F. L. Raymond writes from Yeovil that on June 30, 

 8h. 50m., he observed a magnificent meteor. It was white at first, 

 then orange, and moved rather slowly from due S. to a point about 

 5 degrees above S. E, horizon. The meteor left a short diffused trail, 

 but the exact path was difficult to locate owing to the very strong 

 twilight. The same object was seen by the Hev. S. J. Johnson, of 

 Bridport, who gives the time as 8h. 52m , and says the path was from 

 an altitude of about 15 degrees in due south to about 5 degrees above 

 horizon at a point a little to the eastward. Motion rather slow ; 

 duration 3 seconds. Miss Mary E. Glennie also writes from Diuan, 

 France, that the fireball as seen from that place appeared larger than 

 the moon, and emitted zigzag streams of fire as it went along. A 

 sound like distant thunder was heard about a minute after its disap- 

 pearance. At St. Brieux, France, the bolide passed from W. to E., 



Observers 



