534 



NA TURE 



[April 5, 1900 



generally fogged that the density of a star of the nine- 

 teenth magnitude, or of even nebulosity of the same 

 brightness, is not distinguishable. This glare, therefore, 

 apparently places a limit on the photographic penetrative 

 power of the instrument employed, and, as far as Dr. 

 Roberts's conditions of observation are concerned, the 

 limit for luminosity of the feebleness of about the 

 eighteenth magnitude is reached. Perhaps for such a 

 clear atmo'^phere as is experienced at Areqnipa, in 

 Peru, and like stations, and with instruments of larger 

 aperture, even fainter stars might be reached. This is a 

 subject, however, which requires considerable research 

 before any very definite statement can be accurately 

 made. 



The next point dealt with is perhaps the most impor- 

 tant of all. It is the general impression that if a 

 photographic plate be exposed in a telescope for several 

 hours, it will, on development, show more stellar images 

 than if it had been exposed for one hour ; indeed, the 

 longer the exposure, the more detail will be impressed 

 on the photographic plate, and one can quite imagine 

 that if exposure were sufficiently long, the whole plate 

 would be covered with images, indicating that we are 

 practically surrounded by a wall of stars. 



This, however, is not the case according to the in- 

 vestigations of Dr. Roberts, and he produces very strong 

 evidence in his favour. If two exposures be made on 

 one object, say, one lasting one and one-half hours, and 

 another for twelve hours, and should the same amount of 

 detail be depicted on each, the natural deduction would 

 be that the longer exposure did not show any more 

 detail than the shorter one, because there would be no 

 more images to record. From a minute examination of 

 photographs of the great nebula in Andromeda, in Orion, 

 the group of the Pleiades, and the region of the Milky 

 Way about Cygnus, Dr. Roberts finds that such is the 

 case, and that lengthened exposure need not necessarily 

 mean an increased number of stellar images. He is thus 

 led to accept the fact as a demonstration " of the accuracy 

 of the surmises of astronomers in the past, that the part 

 of the starry universe visible from the earth is limited in 

 extent, and that notwithstanding the enormous assistance 

 afforded by the photographic method, we are again 

 brought to a check because of the inadequacy of the 

 powers we possess to enable us to peer beyond that part 

 of space in the midst of which we are placed. . . ." 



It would be interesting to inquire whether Dr. Roberts 

 has examined other photographs of these regions taken 

 by different observers who have also employed long 

 exposures and other instruments, and, if so, whether his 

 opinion as regards this point has been endorsed. Such 

 an examination as here suggested might lend additional 

 strength to the conclusion he has already drawn. 



In directing attention to the evolution of stellar 

 systems, the author places before his readers a series 

 of beautiful illustrations of his plates showing rich fields 

 of stars of various degrees of condensation ; spiral 

 nebulae varying as regards symmetry ; circular, annular 

 and irregular nebulae ; and lastly, nebulas of a cloud-like 

 nature, which cover enormous areas and are conspicuous 

 by their great irregularity. The wonderful groupings 

 into lines and curves of many of the stars in these 

 NO. 1588, VOL. 61] 



clusters and nebulae, and the forms of the nebulous 

 matter, leave, as Dr. Roberts points out, no room for 

 doubt that they are the effects of physical causes, and, on J 

 account of their persistency on the plates, are very im- 

 probably due to coincidence only. The author further j 

 differentiates between those stars which are actually in- ' 

 volved in nebulte, and those which are situated simply ' 

 in the line of sight, but do not conform with the trend of ' 

 the spirals or with the curves of the nebulous stars j 

 involved in them. j 



Many other points of interest are referred to in these ■ 

 pages, among which we may mention the variability and : 

 motion of nebulae ; these and others, however, we must j 

 leave to those of our readers who have the good fortune j 

 to examine the volume for themselves. 



In the publication of this work, Dr. Roberts has not 

 only nobly enriched astronomical science, but has raised 

 a monument to himself which will last as long as 

 astronomy has any interest for mankind. This handsome 

 book, besides being a most valuable mine of information, 

 serves not only as a demonstration of the success that 

 has rewarded his efforts after an infinite amount of most 

 skilful instrumental adjustment and working, but as an 

 excellent example of the valuable work that can be 

 accomplished single-handed when one is endowed with 

 both the love for and the means of studying the oldest of 

 the sciences. William J. S. Lockyer. 



TWO MONSTROUS REPTILES. _j 



{\) A Complete Mosasaur's Skeleton and (2) A Skeleton ^i^^ 

 of Diplodocus. Being Parts iv. and v. of vol. i. of,(| 

 " Memoirs " of the American Museum of Natural'^ 

 History. By H. F. Osborn. With 8 Plates and 28I 

 Text Illustrations. (New York : The Knickerbocker^ 

 Press, 1899.) -.si 



THE memoirs above-mentioned are the latest of a I 

 series which, though not yet in their second volume, | 

 have already taken their place in the foremost rank of | 

 zoological publications. For this praiseworthy result the'J 

 world is largely indebted to the author of the present ) 

 memoirs, through his great monograph on " The Extinct | 

 Rhinoceroses" — the third in order of succession to ap- ^ 

 pear. That came to those cognisant of his rich resources 1 

 and familiar with his former doings as the fulfilment of ^^ 

 a desire, and in itself set a high standard of excellence. '\ 

 In the memoirs under review this has been fully main- \ 

 tained, both as regards text and illustrations, which are i 

 alike highly finished works of art, worthy a pupil of '., 

 Huxley. The two sets of remains dealt with are equally -, 

 remarkable — one for the fact that parts usually lost by : 

 decomposition after death are here preserved ; the other ; 

 as furnishing us, for the first time in an undisturbed 'j 

 state, with well-nigh half the axial skeleton of a colossus,.! 

 whose backbone was hitherto known only by some few \ 

 isolated vertebras. 



The specimen of the Mosasaur is from the famous \ 

 Kansas Chalk, which vies only with that of Mesvin in yield- \ 

 ing the remains of the later aquatic reptiles, as evidenced | 

 by the grand series preserved in the Brussels Museum. \ 

 The specimen under consideration measures some thirty \ 

 feet in length, and is in detail noteworthy for the con- - 

 dition of its cervical vertebree and limb skeleton, and *i 



