Feb., 1904.] 



KXOWI.I'IK;]' \- SCIHNTIl-lC XI'.WS. 



21 



Minima of Al^ol may be ohserveil 011 tlio isl at 

 11.52 p.m.. 4tli at 8.41 p.m., jtli at 5.30 p.m., jjiid at 

 1.35 a.m., 24th at 10.24 P-m-. and tlie 2jlh at 7.13 p.m. 



Telescopic Objects: — 



Clusters.- -M35, situated about 2 K.K. of >; Gemi- 

 norum or about midway between t Tauri and < Geini- 

 norum. Fairlycompacl, presenting a beautiful appearance 

 of star streams when observed under favourable con- 

 ditions. R..\. \1.^ 3™ Dec. N. 24' 21' N41, about 4" 

 directly south of Sirius; \isible to naked eye; Messier 

 resjistered this group as "a mass of small stars," K.A. 

 \'I.'' 43"> Dec. S. 20^ 38' M44, the Pra-sepe in Cancer, 

 visible to the naked eye as a nebulous patch, best seen 

 and easily resolvable with a pair of opera or field glasses. 

 On account of the scattered nature of the group the 

 cluster eflPect is lost when observed through a telescope 

 unless very low powers be employed. Situated a little to 

 the west and about midway of the line joining <• and 5 

 Cancri. R..\. \'I11.'' 34" Dec. N. 20^ 20'. 



Double Stars. — Castor, separation 5"-8, mags. 2-7, 3-7. 

 E.xcellent object for small telescopes. The brigiitest pair 

 to be observed in this country ; can always be relied upon 

 as a good show object. 



»■ Geminorum, separation6"-3, mags. 4, 8*5; very pretty 

 double. 



i Cancri, separation i"-i, 5"-3, mags. 5-0, 5-7, 5-5; with 

 small telescopes the wider component is readily seen. 



7 Draconis, separation 6i"-j, mags. 4-6, 4-6 ; a pretty 

 and easy double, can be separated by observing with a 

 pair of opera glasses. 



The Showerof November Leonids in 1903. 



To THE Editors of '■ Knowledge." 

 Gentlemen, — .'\t the end of my paper under this heading, 

 published in the J.anuary number of " Knowledge," I referred 

 to a second magnitude meteor seen at 15 hrs. 59 mins. on 

 November 15, and moving in a long and slowly traversed 

 path from a probable radiant at 113 —34 . Two of the 

 obser\ations from which the real course was deduced were. 

 however, somewhat imperfect and indefinite. Fortunate!)' I 

 have since received a description of the object as seen at 

 Greenwich at 15 hrs. 59 mins. jH sees., and I h.ive recom- 

 puted the hei.ghts and radiant. The latter position was 

 really in Hydra at about 147 -ii^ and the height of the 

 meteor varied from gi to 45 miles during its extended flight 

 of 128 miles, which it pursued at the rate of 29 miles per 

 second. The object certainly travelled at a much slower 

 speed than is consistent w ith a parabolic orbit. 



Professor Herschel has recently been comparing the re- 

 corded paths observed on the night of November 15 by several 

 observers, and has foimd a few interesting accordances. 

 Two of these were of brilliant Leonids, and the real courses 

 which I have calculated for these agree very closely with the 

 results previously obtained by Professor Herschel, and are as 

 follows : — 



Date and Greenwich mean 1 November 15 November 15 

 time of the observations. . ) lO hrs. 45.^ mins. 18 hrs. 7 mins. 



Observers 



A. S. Herschel, A S. Herschel. 

 Slough. Slough. 



\V V D.. A. King, 



Bristol. Sheffield 



I W. F v.. 



Bristol 

 Notwithstanding the richness of the Leonid shower in 1903, 

 and the large number of observations, comparatively few of 

 the same meteors appear to have been seen at two stations. 

 Bishopston, Bristol, Yours faithfully, 



January 6, 1904. W. F. Denning. 



Comhuted by F. Siiii.i.ingto.v Sc.\li-:s, imj.m.s. 



Magnification of Objectives and Eyepieces 



I r is scarcely necessary to explain to any worker with 

 the microscope that, whereas a simple lens gives a single 

 magnification only, the essential principle of the compound 

 microscope is that the image formed by the first lens or 

 system of lenses, called the objectixe, is itself again 

 magnified by a second lens or system of lenses known as 

 the eyepiece, or ocular. But, simple as this is in prin- 

 ciple, the means by which it is brought about, and the 

 various points connected therewith, aie often not fully 

 understood by ordinary workers, many of whom are not 

 clear as to the exact meaning of such terms as one inch, 

 half-inch, lVx., as applied to objectives, or to references 

 to angular aperture as compared with numerical aper- 

 ture, aplanatic aperture, Ac. 



Briefly, the principle on which olijectives are rated is 

 as follows: We have here a Itns, or system of lenses, 

 with which we form our first magnified image, and this 

 image is formed at a definite distance from tlie back of 

 the lens. According to Knghsh standards, this distance 

 is 10 inches, which was originally adopted as being the 

 the normal visual distance of the human eye. Then it 

 follows that the relative size of object and image will 

 vary directly as their respective distances from the lens, 

 or rather from its centre. Accordingly, if the two dis- 

 tances are i inch and 10 inches respectively, the initial 

 magnification will be ten times, and here we have our 

 I -inch objective. If the distances are 2 inches and 

 10 inches, the magnification will be five times, and the 

 objective will be known as a 2-inch. If the distances or 

 foci are A incli and 10 inches, the magnification will be 

 twenty times, and the objective is A inch, whilst a i-i2th 

 inch objective magnifies initially 120 times. 



On the Continent, however, the image comes to a focus 

 about 6h inches behind the objective, this being the 

 Continental tube-length, but the rating seems to gene- 

 rally remain the same — the i-inch magnifying 10 

 times at 10 inches, the 2-inch 5 times, and so on. 

 Thus a Continental i-inch objective used with a 

 6i-inch tube should only give an initial magnifica- 

 tion at this distance of 6-5 diameters. As a matter 

 of fact, however, objectives are nearly always overrated, 

 sometimes absurdly so, and therefore a Continental 

 I-inch may give an initial magnification exceeding 10, 

 even with the short tube. 



Of course the tube-length of a microscope can gene- 

 rally be varied, and the result will be in the first place a 

 readjustment of focus and a conseijuent variation in the 

 magnification. But the second result is that, as objec- 

 tives are not meant to be used for uncovered objects, they 

 have been carefully " corrected " for a certain definite 

 thickness of cover-glass. The Royal Microscopical 

 Society has used its powerful influence to bring makers 

 into line throughout the world with regard to the stan- 

 dardizing of the screw of objectives, the diameters of eye- 

 pieces, and the size of sub-stage condensers, and it would 

 be a great advantage if it could also standardize the 

 thickness of cover-glass to which objectives are corrected. 

 Perhaps this may be done some day; in the meantime 



