Nov. 2, 1 871] 



NATURE 



19 



circles of the corona are purple alike where they abut on the 

 junction, and what is the order of colours in the semicircle on 

 receding from the junction. A record as to which is which of 

 the two halves of the biquartz should be carefully preserved. 



Should secondary atmospheric polarisation be so strong as to 

 throw doubt on the results (which may be judged of by noticing 

 the light on the dark moon), it would be well to rotate the ana- 

 lyser till the two halves seen on tlie dark moon are purple alike, 

 and then alter the pointing of the telescope, and repeat Praz- 

 mouski's observation. 



It will be observed that the same general principles apply to 

 the elimination of atmospheric polarisation, whether the polari- 

 scope employed be a Savart's polariscope, a polariscope with 

 quartz wedges, or a biquartz polariscope. 



C. This is of little intrinsic interest, its chief use being to clear 

 up possible doubts as to the results obtained by the observers of 

 A and B. Should there be an observer not otherwise employed, 

 he might be deputed to observe the direction of the Savart's bands 

 on disappearance, both on the dark moon and the surrounding 

 sky, and whether this direction changes during totality. Also it 

 should be specified in which pair of opposite quadrants they were 

 black -centred and in which white-centred. Should this be found 

 impossible or uncertain (the instrument being unprovided with 

 the adjunct mentioned above), the Savart might be used as a 

 simple Nicol by turning it end for end, so that the quartz plates 

 are next the eye ; and with this the plane of polarisation might 

 be roughly determined by means of the azimuth of the principal 

 plane of the Nicol when the light most nearly disappears. 



Should registration of the azimuth be attempted, the Savart 

 would be fixed so as not to be reversible. In that case the ob- 

 server might be provided with a double-image prism and dia- 

 phragm-tube for separate use in case of need. 



Stoppage of s/ray light in a telescope designed for polarisation 

 The want of this appears to have occasioned some difficulty at 

 the last eclipse. 



The simplest way is by a stop, with a hole just large enough 

 to contain the image of the object-glass. Such e.xists in the 

 erecting eye-piece, where an image of the object-glass is formed 

 in the body of the eye-piece. It exists too, in a Gregorian or 

 Cassegrainian telescope, where tiie stoppage is imperative. But 

 in an ordinary refracting telescope, with an inverting eye-piece, 

 the eye-hole (from certain motives of convenience) is larger than 

 in front of (/. e. nearer the object-glass than) the bright circle, or 

 image of the object-glass ; and unless the tube is sufficiently pro- 

 vided with stops, when a faint object near a bright one is looked 

 at, light from the bright, reflected from the inside of the tube, is i 

 liable to enter the field of view. Large instruments are pro- 

 vided with stops ; but I fancy smaller instruments are sometimes 

 turned out without them. This should be looked to. 



The observer may test the correctness of stopping by taking 

 out the eye-piece, inserting a paper disc with a central hole of 

 the size of the field-glass, turning the instrument nearly but not 

 quite to a bright object, as well as to points more distant from the 

 bright object, and noticing whether the side of the tube, even 

 when viewed in a diiection grazing the edge of the hole, is pro- 

 perly dark, so that only the edges of the stops are seen.* On the 

 other hand, the stops should not obstruct a clear view of the 

 object-glass as seen through the hole representing the field- 

 glass, or they will render the outer portions of the object-glass 

 useless. 



General Remarks 

 I consider the observation recommended by Mr. Ranyard (see 

 Nature, Aug. 24, 1S71), very important, if, after what Praz- 

 mouski and Ranyard have done, the point be still deemed doubtlul. 

 Prazmouski's observation seems to have been beautifully devised 

 and executed, but carelessly described. It is only by conjecture 

 that I can make sense and harmony with what is known, out of his 

 observations as described by himself. But I think that Mr. Ran- 

 yard has at least shown that our conjectural interpretation of 

 Prazmouski's observation is the right one ; and if so, the point 

 seems settled. 



It is for this reason that, in lieu of No. 3, first half, I proposed 

 something new. What becomes of the magnesium, &c., which 

 the spectroscope reveals low down in the gigantic puffs which the 

 sun emits ? '1 he hydrogen must surely carry the magnesium, 

 c&c, with it to the higher regions, though the magnesium, &c. , 

 would soon be condensed, and so would not be detected by the 

 spectroscope. These substances would exist in the form of an 



'\ If reflection occurs from the part of the tube so near the eye as not to 

 appear ivithin the field, it will not signify much. 



exceedingly fine haze or dust. I use the two words, " haze " to 

 denote a filmy cloud of molten "dust" of solid matter. This 

 haze or dust is capable of detection, and, according to my inter- 

 pretation, //(7j-been detected, by polarisation ; and it is interesting 

 to know how low down it can be detected. Mr. Stoney's specu- 

 lations as to layers are utterly inapplicable here, as they imply a 

 state of tranquillity quite unlike what we now know to exist, at 

 any rate in connexion with the puffs. 



I don't know why, in the second half of No. 3, Mr. Ranyard 

 prescribes placing the line of junction across a sector or rift, if 

 by that he means turning the eye-piece carrying the quartz plates 

 ■so that the Une is perpendicular with the corona to the sector. 

 It would be more likely to yield results if it cut il obliquely, as 

 represented for the corona in Fig. 2. But proliably he only 

 means pointing the telescope so that the junction cuts the rift. 

 If the observer notices contrasting colours, he may then proceed 

 to determine the plane of polarisation, G. G. S. 



SCIENTIFIC SERIALS 

 The Journal of the Quekett Microscopical Club. No. 16. Oc- 

 tober 1871. "Microscopic Work and Conjectural Science," 

 being the address of the President ( Lionel S. Beale, M. B. , F. R. S. ), 

 for the year 1871. This address is chiefly occupied in combating 

 the method, presumed to have been adopted, of depreciating one 

 kind of scientific investigation in order to elevate another, and 

 attacks without ceremony those who would elevate physical 

 science to the disparagement of microscopical observation. — " On 

 the Examination of the Surface Markings of Diatoms by the 

 Oxy-calcium Light," by N. E. Green. The writer of this paper 

 details his examination of such diatoms as Isthmia, Biddulphia, 

 Triceratium, Pleurosigma, &c., as opaque objects by high 

 powers, as one-sixth Ross and one-twelfth Gundlach, through the 

 agency of the oxy-calcium light. The conclusion at which he 

 has arrived is, that the markings on all the above, except Pleuro- 

 sigma, resemble " craters," the surface "being studded with rows 

 of small shallow craters, the sharp edges of which projected 

 slightly above, while the centres seemed to be below the surface." 

 In Pleurosigma a different structure of the surface was observed. 

 " The lime light brought out most distinctly the bead-like charac- 

 ter of its markings ; they stood out in bold relief like rows of 

 Indian corn." — The Inaugural Address of the South London 

 Microscopical and Natural History Club, by R. Braithvvaite, 

 M.D., F. L.S., is principally devoted to suggestions on the vast 

 field for observation at the disposal of the microscopist. — "On 

 Nucleated Sporidia," by M. C. Cooke, M.A. After describing 

 the general structure which prevails in the genus Peziza of As- 

 comycetous Fungi, the writer details his method of mounting 

 sections for the microscope in pure glycerine. The nucleated 

 sporidia, so prevalent in this genus, are affirmed to be so affected 

 by this method that in a short time all traces of the nuclei are 

 lost, and the object of the paper is to indicate the doubtful value 

 of nucleated sporidia in specific characters. The true nature of 

 such nuclei and their uses are said to be obscure. 



In the Reiue Scientifiijue, Nos. 13 — iS, are many valuable 

 articles. Further reports are given of the proceedings of the 

 Edinburgh meeting of the British Association, and a translation of 

 Prof. T. Sterry Hunt's address to the Indianopolis meeting of the 

 American Association. We have also a memoir of M. Lartet 

 by M. G. de Mortillet ; Helmholtz's paper on the rapidity of 

 propagation of electro-dynamical actions ; report of M. Chau- 

 veau's lectures on the physiology of virulent maladies ; a lecture 

 by M. Claude Bernard on the method and principle of physio- 

 logy ; a translation of P. Secchi's paper on the solar protuberances 

 from the Atti dell' Acadeinia ponitificia de niioz'i Lined ; a bio- 

 graphical sketch of Haidinger by M. Fouque ; reports of the 

 proceedings of the various scientific institutions in France and 

 Belgium ; and translations of lectures delivered at the Royal 

 Institution, University of Edinburgh, &c., by Prof. Tyndall, 

 Dr. Carpenter, Dr. Laycock, and others. 



SOCIETIES AND ACADEMIES 



Paris 

 Academy of Sciences, October 23.- — The greater part of the 

 communications read at this meeting were devoted to chemical 

 subjects. Of mathematical papers only one was presented— 

 namely, a continuation of M. Chasles' memoir on the determina- 

 tion of a series of groups of a certain number of points on a 

 geometrical curve. — A note was read by M. J. Bertrand on the 



