286 



KNOWLEDGE. 



[December, 1901. 



connects with it and actuates it. A reference to the accompanying 

 illustration will make the matter clear :^A is a sleeve in which 

 the upper part of the frame is supported and can he moved 

 forward or backward. Bis the clamping screw with which it is 

 held in po.sition. 



It may be mentioned that two or more milled heads of 

 different sizes will often be found useful on the camera rod, for 

 they will permit of the rate of movement being increased or 

 diminished as may be desired. 



A Xew SuiiSTAOE Condenser. — Several writers have 

 advocated the use of a good low-power Substage Achromatic 

 Condenser, but nothing that really filled the gap was obtainable 

 until the introduction of Mr. C. Baker's new condenser, referred 

 to in the April number of KNOWLEHfiE. Zeiss's Achromatic 

 Condenser was right so far as it went, but it had several dis- 

 advantages. It was large — too large — and restricted the move- 

 ments of the mechanical stages, also it yielded an aplanatic 

 aperture of less than '70, yet the fact of its having a power of 

 A-inch caused it to be appreciated, and largely used. Messrs. 

 Watson & Sons have just introduced a new condenser having 

 a power of -j-'^-inch, and a numerical aperture of •'.19, which is 

 practically totally aplanatic. This will be found a great 

 auxiliary to the range of condensers, especially as it possesses a 

 sufficiently large field-lens, and is not so bulky in the mounting 

 as to obstruct the moving plates of the mechanical stage. 

 There is quite a comfort in using a condenser of this power 

 with objectives of low and medium magnification. 



An Experimental Offer. — There are many microscopists 

 possessing experience who are interested in every new develop- 

 ment, and to such, an opportunity for practically testing new 

 apparatus would be acceptable and valuable. I have one of 

 these new condensers of Watson's in my possession, and 1 am 

 willing to send it for trial to the first four applicants who are 

 regular subscribers of Knowledge who requisition it. I will 

 only stipulate that it shall not be retained more than four days, 

 and returned carefully packed. If this experiment is a])preciated, 

 it may be possible to make similar offers concerning new items 

 of apparatus that may be described from time to time, should 

 the makers of apparatus be willing to co-operate. I should be 

 glad to receive suggestions and expressions of opinion on the 

 extension of this scheme. 



Oii.iECT Marker. — A very neat accessory, and one that is 

 frequently required, is a means of indicating by a mark on the 

 specimen cover exactly where a particular structure that has 

 been observed can be found again. Leitz makes such a fitting 

 which replaces the objective after the required portion of the 

 object has been found. The front portion of the marker is 

 then brought into contact with the object, and a mark is made 

 ui)on it which indicates for the future the position of the 

 particular structure. Its cost is very small. 



NOTES AND QUERIES. 



J. Oi/IIric. — If you want the very best i-inch objective that 

 can be obtained you must have Zeiss's Apochromatic. The 

 Holoscopic lenses of Watson's are not so well corrected for 

 colour as the apochromatics, but are exceptionally good, being 

 very free from spherical aberration. As you already appear to 

 have a very good lens of this power, the better way would be to 

 arrange with the makers for an 0])portunity of examining their 

 objectives, and you could then decide whether or no you would 

 benefit by having a new lens. I can give you no information 

 about the other person you name. 



,/. E. — The J in. 1) mm., and ^ in. 4 mm. apochromatic objec- 

 tives by Zei.ss are certainly superior to all other lenses that 

 are made. To derive benefit from them experience and .skill 

 are necessary ; presuming that you possess this you would 

 certainly do better with either of these than with the other that 

 you already have, although that is a very good lens. You have 

 to remember that special compensating eyepieces have to be 

 used with apochromatic objectives. You have nothing to fear 

 on the score of oxidation, the material now used is quite safe and 

 permanent in the United Kingdom. 



W. Jl. /*. M. — I fully appreciate the value of your suggestion, 

 which has, however, been anticipated. A further reference to 

 the subject will be made in the next issue. 



Communications and enquiries on Microscopical matters are 

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

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



NOTES ON COMETS AND METEORS. 



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



Comets and their Discoveet, — Brief reference was made lust 

 month to the exceptional dearth of comets in 1901. Whether this 

 rarity is to be attributed to actual scarcity or to inadequate searching; 

 for these bodies is not clear, for we have no .statistics bearing directly 

 on the work and its results. That certain observers are engaged in 

 specially swecpin;; for new comets we know by occasional announce- 

 ments of their success, but we have no figures showing the extent to 

 which these labours are pursued. Quite possibly there has recently 

 been a want of thoroughness in the quest for these bodies ; if so, it 

 would fully account for their comparative absence in our nocturn.al 

 skies. Only one new comet has been observed during the present 

 year, and only one of the many periodical comets known has been 

 re-detected. It is true that the comet which suddenly appeared at 

 the end of April and formed such a brilliant appearance in the 

 southern hemispliere must be considered quite a notable object of its 

 class, and it made ample aniends for other shortcomings in the 

 number of cometary phenomena ; but it was scarcely visible at all in 

 the northern hemisphere. Tlie only other comet seen was Eneke's and 

 this faintly and inconveniently displayed itself in the morning twiligbt 

 of August and September, so that very few, if any, observers in this 

 country saw anything of it. Practically, therefore, there has been 

 nothing in the world of comets for .northern observers to see during 

 the present year. Yet these bodies are, as Kepler said, " as numerous 

 as the fishes in the sea," and the late Dr. Kleiber found from a 

 theoretical investigation that the mean number of comets always 

 )ireseut within the limits of the solar system is .5934 ! It is a matter for 

 regret that comet seeking is comparatively little pursued in England. 

 The climate, it must be confessed, is not favourable for this or indeed 

 any kind of aEtrouomical work, but Caroline Herschel more tlian a 

 century ago proved wliat could be accomplished in this attractive 

 branch of observation with a very small telesco])e in able and jiatient 

 hands. There is really no effective reason why English observers 

 shoidd not furnish a fair proportion of cometary discoveries, and tlie 

 fault is to be found more in want of inclination and effort than in 

 lack of opportunity. No doubt comet seeking, to he productive, 

 demands a large amount of systematic and persevering application, 

 and amateurs are seldom in a position to devote themselves to it with 

 the necessary thoroughness. Many of them undertake a little sweeping 

 at odd times, but are led to relinquish the work after some unsuccessfiil 

 attempts. Sometimes this is brought about by the great prevalence of 

 nebulae, which form a serious hindrance to expeditious sweeping and 

 occasion many ditliculties, due to the necessity for ascertaining positions 

 and making troublesome references and comparisons, so that the 

 various objects may be identified whenever they are encountered. In 

 Virgo, Coma Berenices and Ursa Major nebuhv present themselves in 

 almost every sweep, and the observer is apt to get bewildered, and often 

 finds it impossible to proceed satisfactorily. In many other parts of 

 the firmament, however, conspicuous nebulce are rare, and sweeping 

 may be done with little interi'uption. The work becomes very enter- 

 taining after a little practice has smoothed away the difficulties, for 

 the prospect of making a discovery is always alluring, while the very 

 interesting objects which pass through the field serve to maintain the 

 interest. 



Showers or Telescopic Meteors in the Battime, — Nearly 

 every year brinjs us an additional record of a daytime shower of 

 telescopic meteors (?). Mr. J. Coles, of Liphook, Hants., has just 

 puhlislied an account of an event of this kind which he witnessed on 

 the afternoon of September 18th last, while viewing Areturus in a 

 7-in. Newtonian reflector. He says the meteors crossed the field from 

 W. to E. at the rate of twelve a minute. A large number of descrip- 

 tions might be quoted from scientific journals relating to similar 

 showers. Thus in the Astronomical Reijister (October, 1S70) Rlr. C. 

 Grover, of London, says that on July 24th, 1870, at lOh. l.jm. a.m., 

 while examining Mercury, then close to the sun, luminous objects 

 we.*e seen passing through the field. Sometimes ten were counted at 

 the same instant. They varied in size from mere points to bodies 

 fully equal the apparent diameter of Jupiter, and had sharply-defined 

 discs, hut a few were irregular. They were passing for half an hour. 

 He used a 12 j in. equatorial reflector. Mr. W. K. Brooks, in the 

 Sidereal Messenger (\'ol. 11., No. 10). says that on November 28th, 

 188.3, while sweeping about Itf over the sunset point, he observed a 

 wonderful shower of telescopic meteors. Most of them left a faint 

 train visible in the telescope for one or two seconds. The motion of 

 the majority was to the northward, but with an occasional group 

 moving southward. 



There is every reason to suppose tliat in these and other cases the 

 objects observed were of purely terrestrial origin, and of so light a 

 texture that thev had risen to a considerable height and were carried 



