94 



NA TURE 



[November 24, 1892 



is invariable in tint and disappears by inclination of the body. 

 Such instances are no real exceptions, but are due to the 

 reflecting plates being curved, or having pigmentary matter 

 beneath them, or an opalescent medium above them. In this 

 way some of the most extraordinary and beautiful colour effects 

 it seems possible to conceive are produced. 



In examining objects with the perforated mirror a single light 

 is necessary. The sun is of course the best, and the electric 

 light probably almost as good. I frequently employ the lime- 

 light, but a good parafiin lamp may be used as a substitute. 

 Ordinary gas is unsuitable. The light should be placed in front 

 of the observer, its direct rays being prevented from fallirg on 

 the objects by means of a book or partition of some kind resting 

 on the table, and of such a height that the light can be seen 

 above it. On placing the mirror to the eye the light may be 

 reflected from the mirror on to the object, and the latter manip- 

 ulated so as to reflect the ray back through the perforation in 

 the mirror to the eye. The incidence is thus known to be 

 normal, and the colour observed is the one to be recorded. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 

 Cambridge. — The following letter has been addressed by 

 the University of Cambridge to that of Padua, which is about 

 to celebrate the tercentenary of Galileo's professorship : — 



Universitas Cantabrigiensis Univcrsitati Patavinae S.P.D. 



Litteras vestras, viri doctissimi, Galilaei Galilaei Pro- 

 fessoris vestri celeberrimi in laudem conscriptas vixdum nuper 

 perlegeramus, cum slatim in mentes nostras rediit non una 

 Italiae regio viri tanti cum memoria in perpetuum consociata. 

 Etenim nostro quoque e numero nonnulli urbem eius natalem 

 plus quam semel invisimus, ubi Pisano in templo lucernam 

 pensilem temporis intervallis acquis ultro citroque moveri adhuc 

 iuvenis animadvertit ; etiam Vallombrosae nemora pererravimus, 

 ubi antea scholarum in umbra litteris antiquis animum puerilem 

 imbuerat ; ipsa in Roma ecclesiam illam Florentinam intravimus, 

 ubi doctrinae suae de telluris motu veritatem fato iniquo abiurare 

 est coactus ; Florentiae denique clivos suburbanos praeterivimus, 

 ubi provecta aetate caeli nocturni sidera solus contemplabatur, 

 ubi exfrema in senectute diei lumine orbatus cum Miltono 

 nostro collocutus est, ubi eodem demum in anno mortalitatem 

 explevit, quo Newtonus noster lucem diei primum suspexit. 



Hodie vero ante omnia non sine singulari voluptate sedem 

 quandam doctrinae insignem, intra colles Euganeos urbemque 

 olim maris dominam positam recordamur, ubi trecentos abhinc 

 annos saeculi sui Archimedes discipulorum ex omni Europae 

 parte confluentium numero ingenti erudiendo vitam suam 

 maturam maxima cum laude dedicavit ; ubi, ut Livii vestri 

 verbis paulum mutatis utamur, ultra colles camposque et flumen 

 et assuetam oculis veslris regionem late prospiciens, caelo in 

 eodem, sub quo vosmet ipsi nati estis et educati, instrumento 

 novo adhibito inter rerum naturae miracula primus omnium 

 Lunae faciem accuratius exploravit, lovis .satellites quattuor 

 primus detexit, Salurni speciem tergeminam primus observavit, 

 ultraque mundi orhem ingentem a Saturno lustratum fore 

 suspicatus est ut etiam alii planetae aliquando invenirentur. 



Ergo vatis tarn veracis, auguris tam providi in honorem, nos 

 certe, qui Professorum nostrorum in ordine planetae etiam 

 Saturno magis remoti ex inventoribus alterum non sine superbia 

 nuper numerabamus, hodie alterum ex Astronomiae Professoribus 

 nostris, Georgium Darwin, nominis magni heredem, nostrum 

 omnium legatum, quasi Nuntium nostrum Sidereum, ad vosmet 

 ipsos libenter mittimus. Vobis autem omnibus idcirco gratu- 

 lamur quod turn Italiae totius, tum vestrae praesertim tutelae 

 tradita est viri tanti gloria, qui divino quodam ingenio praediius 

 rerum naturae in provincia non una ultra terminos prius notos 

 scientiae humanae imperium propagavit quique caeli altitudmes 

 immensas perscrutatus mundi spatia ampliora gentibus patefecit. 

 Valete. 



Datum Cantabrigiae 



a. d. viii Kal Decembres 

 A. S. MDCCCXCII. 

 Mr. F. Darwin has been appointed Deputy Professor of Botany 

 for the current academical year. Prof. Babington being unable 

 to lecture on account of the state of his health. 



NO. 



204, VOL. 47] 



SCIENTIFIC SERIALS. 



American Journal of Science, November. — Unity of the 

 glacial epoch, by G. Frederick Wright. An examination of 

 the evidence in favour of two successive glacial epochs separated 

 by an inter-glacial epoch, during which the glaciated area was 

 as free from ice as it is at the present time. This evidence is 

 shown to be inconclusive, at least as (ar as American observa- 

 tions go. — A photographic method of mapping the magnetic 

 field, by Charles B. Thwing. Iron filings are strewn upon the 

 film side of a dry plate laid horizontally in a magnetic field, and 

 the plate is exposed to light from above. The filings are then 

 brushed off, arid the plate developed. From the negatives excel- 

 lent lantern slides maybe obtained. — Contributions to mineral- 

 ogy, No. 54, by F. A. Genth, with crystallographic notes by S. L. 

 Penfield. Description and analysis of aguilarite, metacinna- 

 barrite, dollingite, lutile, danalite, yttrium-calcium fluoride, 

 cyrtolite, lepidolile, and fuchsite. — The effects of self-induction 

 and distributed static capacity in a conductor, by Frederick 

 Bedell, Ph.D., and Albert C. Crehore, Ph.D.— The quantita- 

 tive determination of rubidium by the spectroscope, by F. A. 

 Gooch and J. I. Phitmey. The method is that of comparing 

 photometrically the intensity of a certain line in the spectrum of 

 the metal under investigation with the intensity of that given by 

 a standard solution coniaining a known amount of the metal. A 

 definite amount of the salt solution— usually the chloride — is 

 taken up by a hollow coil of platinum wire, which may be made 

 to take up constant quantities of liquid by taking care to plunge 

 the coil while hot into the liquid, and removing it with its axis 

 inclined obliquely to the surface. The coils were made of pla 

 tinum wire o '32 mm. in diameter, wound in about thirty turns 

 to a spiral I cm. long by 2 mm. acros-;, and twisted together at 

 the ends to form a long handle. ][ach coil held o"02 gr. of 

 water. With such a coil, the blue rubidium lines were produced 

 in a Muencke burner from 0'0002 mgr. of the chloride. Some 

 test experiments showed that in the case of pure solutions of 

 rubidium chloride the percentage could be found spectroscopic- 

 ally up to I part in 30,000 with an error as low as i"25 per 

 cent. In presence of potassium or sodium, however, the error 

 may be as great as 20 per cent. — Notes on the meteorite of 

 Farmington, Washington County, Kansas, by H. L. Preston. — 

 A note on the cretaceous of North-western Montana, by 11. 

 Wood. — A deep artesian boring at Galveston, Texas, by R. T. 

 Hill. — Notice of a new Oriskany fauna in Columbia County, 

 New York, by C. E. Beecher, with an annotated list of fossils, 

 by J. M Clarke. — Description of the Mount Joy meteorite, by 

 E. E. Howell. — Influence of the concentration of the ions on 

 the intensity of colour of solutions of salts in water, by C. E. 

 Linebarger. 



SOCIETIES AND ACADEMIES. 

 London. 



Royal Society, November 17. — " Stability and Instability 

 of Viscous Liquids," by A. B. Basset, F.R.S. (Abstract.) 

 — The principal object of this paper is to endeavour to obtain 

 a theoretical explanation of the instability of viscous liquids, 

 which was experimentally studied by Professor Osborne 

 Reynolds. 1 



The experiment, which perhaps most strikingly illustrates this 

 branch ot hydrodynamics, consisted in causing water to flow from 

 a cistern through a long circular tube, and by means of suitable 

 appliances a fine stream of coloured liquid was made to flow 

 down the centre of the tube along with the water. When the 

 velocity was sufficiently small, the coloured stream showed no 

 tendency to mix with the water; but when the velocity was 

 increased, it was found that as soon as it had attained a certain 

 critical value, the coloured stream broke off at a certain point 

 of the tube and began to mix with the water, thus showing that 

 the motion was unstable. It was also found that as the velocity 

 was still further increased the point at which instability com- 

 menced gradually moved up the tube towards the end at which 

 the water was flowing in. 



Professor Reynolds concluded that the critical velocity W 

 was determined by the equation 



Wap/fjL'd n, 



where o is the radius of the tube, p the density, and /x the 



viscosity of the liquid, and n a number ; but the results of this 



I Phil. Trans. 1883, p. 935. 



