April 19, 1900] 



NA TURE 



597 



OUR ASTRONOMICAL COLUMN 

 CoMi'OUND Prism of Uniform Dispersion. — Messrs. 

 C. G. Abbot and F. E. Fowle, of the Astrophysical Observ- 

 viry at the Smithsonian Institution, have been investigating the 

 -sibility of obtaining a combination of glasses, the relative 

 versions of which would enable a compound prism to be 

 made having a uniform dispersion similar to that given by a 

 diftraction grating {Astrophysical Journal, xi. pp. 135-139). 

 Their initial experiments were suggested by finding that a 

 parallel-sided combination of three prisms, of which the central 

 one was rock salt and the outer ones glass, gave a much more 

 uniformly dispersed spectrum than either prism used separately. 

 Sample prisms of various kinds of glass were then examined, 

 and after finding two with considerably varying dispersions, 

 prisms were made of such calculated angles that the irregularities 

 of dispersion would be partly eliminated when the prisms were 

 combined in opposite sense, i.e. the base of one to the apex of 

 theother. Taking two such prisms, of 5° 10' and 20° angle respec- 

 tively, and allowing the incident light to first enter the thick 

 prism, then after passing through this and the adjoining thin 

 one, to be reflected back from the last face of the thin prism 

 over its previous course, it was found that the combination had 

 an extremely regular dispersion, and although the wave-length 

 curve shows two points of inflection, throughout the remaining 

 portions long stretches could be selected where the dispersion 

 IS practically uniform. It is thought that for only moderate 

 dispersions the uniformity found will be such as to render 

 corrections unnecessary when comparisons are being made with 

 grating spectra. 



Dynamical Criticism of the Nebular Hypothesis. — 

 In the Astrophysical Journal, vol. xi. pp. 103-130, Mr. F. R. 

 Moulton discusses at some length the bearing of modern dyna- 

 mical treatment on the various problems involved in the nebular 

 theory of cosmic evolution enunciated by Laplace. The various 

 criticisms put forward may be grouped into three categories : (i) 

 comparisons of observed phenomena with those which result 

 from the expressed or implied conditions stated by the hypo- 

 thesis ; (2) discussion of the question whether the supposed 

 initial conditions could have developed into the existing system ; 

 (3) comparisons of those properties of the initial system with 

 the one now existing, which are invariable under all changes 

 resulting from the action of internal forces. 



Under the first section of the discussion it is pointed out that 

 the fact of the planes of the planetary orbits presenting con- 

 siderable variations among themselves, and also that four satel- 

 lites revolve in planes making practically right angles with the 

 average plane of revolution of the system, are in direct contra- 

 diction with one of the chief deductions from the hypothesis. 

 Other objections concerning observed phenomena are the un- 

 accountable and suspiciously irregular distribution of the masses 

 of the planets, and the unexplainable anomaly in the motion of 

 the inner ring of Saturn. 



The objections considered under the second category are that 

 the lighter elements would have escaped from the mass ; that 

 matter would have been detached continually instead of in rings 

 at rare intervals ; that if a ring were contracted into a planet 

 except an infinitesimal remainder distributed along its path, the 

 process of aggregation could not complete itself; that the gravit- 

 ation between the masses occurring in the rare media would be 

 so feeble that they would seldom come in contact, and that 

 Roche's limit and a similar new criterion show that fluid masses 

 of the density which must have formerly existed would be dis- 

 integrated by the disturbing action of the sun. 



In the third section of the inquiry the question of conserv- 

 ation of moment of ntoinentuni is alone considered, but the 

 results obtained are in such discordance with those required by 

 the hypothesis as to indicate that the original nebulous mass, 

 so far from being in any sense homogeneous, was heterogeneous 

 to a degree hitherto considered improbable. 



Involved in the validity of the above statements is the 

 question of the age of the earth, which has been calculated on 

 the theory of the sun's contraction from a gaseous sphere 

 arranged in concentric envelopes. 



The Cape Star Catalogue for 1890.— We have lately 

 received from Dr. Gill, Her Majesty's Astronomer at the Cape 

 of Good Hope, a copy of the catalogue compiled from observa- 

 tions made at the Royal Observatory. The measures were made 

 with the Cape transit circle during the years 1885- 1895, all 

 being reduced to the mean equinox, i890'0, without proper 



NO. 1590, VOL. 61] 



motion. Until June 2, 1889, observati<?ns of all stars were made 

 by the " Eye and Ear" method, but' on that date the chrono- 

 graphic method of recording was introduced, and since that time 

 has been included for all stars except those within 10° of the 

 Pole. Except in a few cases, a reversing prism has been gener- 

 ally used, thus eliminating any error due to the direction of 

 the star's apparent motion through the field of the eye-piece. 

 In the determination of the declinations, evidence of considerable 

 wear was found in the brass screws of the micrometers. These 

 were replaced by new ones of steel, and, as a further precaution, 

 three of the six micrometers were rearranged so as to reverse the 

 direction of the readings. The discussion of the ten years' ob- 

 servations with steel screws shows that the non-periodic cor- 

 rections are still very marked, but that the effects of wear are 

 practically eliminated by the plan of reversing the alternate 

 microscopes. 



In addition, the declinations have had to be corrected for 

 flexure, refraction, and change of latitude, this latter being 

 taken from Albrecht. Tables of the flexure and latitude vari- 

 ations are given. 



The transit circle with which the observations comprising 

 the present catalogue were made is non-reversible, and will 

 be in future exclusively utilised for zone observations, a new 

 instrument being in course of construction for fundamental 

 work hereafter. Considerable pains have been taken to in- 

 vestigate the degree of error introduced by variations in 

 magnitude. No sensible systematic error in declinations is 

 traceable to this source, but in right ascension it is found 

 that the average observer measures the transits of faint stars 

 too late as compared with bright stars, and it is emphasised 

 that in all future catalogues of precision this personal error 

 depending on magnitude should be carefully determined for 

 all the observers. 



The catalogue proper consists of the positions for 1890 of 

 3007 stars, each being designated by its Cape number, and 

 its respective numbers in the catalogues of Lacaille, Bradley, 

 Piazzi, British Association, and Gould. After the positions 

 the corrections are given for annual precession, secular variation 

 and annual proper motion. 



The volume closes with three appendices giving comparisons 

 with other catalogues, special observations of a Canis Majoris, 

 a Canis Minoris, B Centauri and Oj, 0.2 Centauri, owing to these 

 stars having companions of considerable mass, and a discussion 

 of the places and proper motions of twenty-four circumpolar 

 stars used at the Cape for determinations of azimuth. 



FLINT IMPLEMENTS FROM THE NILE 

 VALLE V. 

 'y HE latest number of the Bulletin of the Liverpool Museums 

 -'■ contains a profusely illustrated paper, by Dr. H. O. Forbes, 

 on a collection of stone implements from the Nile Valley, made 

 by Mr. Seton-Karr in 1896, and purchased for the Mayer 

 Museum. The great bulk of the collection was made in Wady 

 el Sheikh, a tributary of the Nile, opening from the south-east 

 into the mud-plain of the river opposite El Fent, which is 

 situated half-way between the stations of Feshn and Maghagha 

 on the railway from Cairo to Assiout. 



The material of which the implements are made is chiefly a 

 yellowish-brown or pale grey, opaque, earthy chert, and is but 

 rarely of the translucent chalcedonic variety from the chalk of 

 England. The collection contains a large number of types 

 which may be classed as bracelets, axe-like tools, leaf-shaped 

 flints, knife-like instruments, hoes or agricultural implements, 

 fabricators, scrapers, cores and flakes, and nondescript stones. 

 The bracelet series shows all the stages in the manufacture of 

 these delicate ornaments, and proves that the suggestion of 

 General Pitt-Rivers, who figured two complete similar ex- 

 amples in the Journal of the Anthropological Institute in 1881, 

 is probably incorrect, that they were formed from " morpholites," 

 or siliceous sphoeroid-shaped bodies occurring in the marine 

 limestone, encircled by a belt or ring, which is divided from the 

 main body only by a thin partition, while sometimes the ring 

 alone is found. The series figured in the paper shows that a 

 flat disk of flint was first prepared, and that this was then 

 perforated by a dextrous stroke of a chisel, and the opening 

 gradually enlarged till sufficiently wide to admit the hand. Of 

 the axe-like tools of which nine, and the knife-like instru- 

 ments of which sixteen, are illustrated, several specimens 



