December 27, 1894] 



NATURE 



201 



"The Elements of Quaternions." 



In answer to my reviewer's question {zide p. 154), I must 

 rankly admit that 



^a) Eq. S ->. 40, should have been a group of six equations, 

 ■ = \ —I, y ^ V - I, &c. ; and that 



{b) The inference should have been that /,/, &c., are (unequal) 



(uare roots of negative unity. H. W. L. HiME. 



THE LICK OBSERVATORY. 



THE recent issue of volumes ii. and ill. of the 

 " Publications" of the Lick Observatory serves to 

 j;ive some indication of the growing activity of this world- 

 famed institution, and to foreshadow the great part 

 which it is destined to play in the astronomy of the future. 

 As in the case of so many other observatory publications, 

 these volumes contain much with which the various 

 astronomical journals have already made us familiar, 

 and one of their chief objects appears to be to collect 

 the observations into a convenient form for reference. 



\'olume ii. is entire!)' devoted to the magnificent 

 micrometric work on stars and nebula: performed by Mr. 

 liurnham during his four years' connection with the 

 Observatory, which, to the general regret, terminated in 

 June 1892. It will be a matter of satisfaction to all 

 interested in the progress of astronomy to learn .hat this 

 keen-sighted astronomer has nothing but praise for the 

 great telescope. He says : " It has more than satisfied 

 the severest tests which could be applied, and the highest 

 expectations concerning its performance have been 

 realised. It is a monument of the genius and skill of 

 the unrivalled opticians, Alvan Clark and Sons, to whom 

 the progress of astronomical work all over the world is 

 so largely indebted." The fact that powers up to 2600 

 have been successfully employed, further emphasises the 

 excellence of the objective. 



Mr. Burnham strongly insists upon the advantages to 

 be gained by the use of a micrometer in which the 

 wires are bright on a dark field. With this method of 

 illumination, he tells us, "any object that can be seen 

 under any circumstances, however faint, can be well and 

 accurately measured. There is no such thing as a star 

 too faint for measurement, if it can be seen at all." 



Besides the immense number of numerical results, the 

 volume gives a mass of most interesting information 

 relating to the various objects observed. Some of this I 

 has already been published, but many new points have 

 been added. Thus, it appears that the observations of 

 ^lOrionis show that " the six principal stars are absolutely 

 fixed with reference to each other, so far as any change 

 is concerned which could be detected by observations 

 covering more than half a century. ' The fulness of the 

 account of this remarkable group, and of the numerous 

 supposed discoveries of stars within the trapezium, furnish 

 an excellent example of the thoroughness which is so 

 characteristic of Mr. Burnham s work. With reference 

 to the very faint star discovered within the trapezium by 

 Mr. .Mvan E. Clark soon after the telescope was erected, 

 he writes : " It is a difficult object with the 36-inch, and 

 certainly has never been seen before, notwithstanding 

 the numerous alleged discoveries with telescopes down 

 to three or four inches aperture. Not less than a dozen 

 of these imaginary stars have been distributed about the 

 interior of the trapezium." 



To the average astronomer, the star 95 Ceti would pro- 

 bably not be of absorbing interest, but to Mr. Burnham 

 it is " the most mysterious and strange double star in the 

 heavens." The companion was discovered by Clark 

 with a 7?. inch, was subsequently measured by Dawes 

 in 1854, and by Burnham with some difficulty in 1SS8, 

 since when he has not been able to see it even with the 

 36-inch. 



Mr. Burnham finds that " none of the stars which 

 have been supposed from spectroscopic observations to be 



NO. 13 13. VOL. 51] 



close doubles, have shown any evidence of the fact when 

 examined with the large telescope under the most favour- 

 able conditions." He then goes on to say that " it is 

 possible some other explanation will be found for the 

 recurrent phenomenon first discovered by Miss Maury in 

 the Harvard spectrum photographs. At all events, it is 

 hardly worth while, until the method has been verified 

 upon some of the numerous known pairs suitable 

 for this purpose, to consume the valuable time of the 

 great telescope in a further examination of objects of this 

 class." One would almost imagine that Mr. Burnham 

 had failed to grasp the fact that the separation of the 

 component stars in such cases, by the spectroscopic 

 method, is solely due to their relative velocity, which in 

 ordinary pairs is relatively small. At any rate, it has 

 been estimated that a telescope of sufficient dividing 

 power to separate the components of /ij Aurigae must 

 have an aperture, not of three, but of eighty feet ! 



Limitations of space forbid further reference to the 

 rich feast which Mr. Burnham has provided ; the value 

 of much of his work will probably be only fully realised 

 by astronomers of another age, but at the same time a 

 large proportion of his results are of the greatest im- 

 mediate interest and value. 



Vol. iii. of the "Publications" consists of Prof. 

 Weinek's now well-known selenographical studies ; a 

 report on specimens of glass similar to those used in the 

 construction of the great object-glass ; an investigation 

 of the glass scale of the measuring engine ; and Prof. 

 Keeler's observations of the spectra of nebulae. 



It comes as a surprise to us to learn from Prof. 

 Holden's introduction to this volume, that the work of 

 the Lick Observatory is not without danger of suffering 

 for want of funds. Even so small a matter as a suitable 

 instantaneous shutter "could not be constructed until 

 the summer of 1893, for lack of funds and of skilled 

 workmen." In the early stages of the lunar photographic 

 studies, we are also informed that the work would have 

 been seriously interrupted had not the Smithsonian Insti- 

 tution come to the rescue with " several small appropria- 

 tions of money." The appearance of the present volume 

 has been made possible by the generosity of Mr. Walter 

 W. Law, of New York City, in providing funds to cover 

 the whole cost of producing the fifteen magnificent plates 

 of the moon which embellish its pages. They are 

 modestly described as " a gift to science," and they 

 afford another example of the practical sympathy with 

 astronomical inquiries displayed by so many of our 

 American cousins. 



Few will be inclined to deny the great value of the 

 lunar photographs which have been taken at the Lick 

 Observatory, and it is a matter for congratulation that 

 the astronomical world has so soon been made acquainted 

 with the first fruits of their investigation. 



Prof. Holden tells us that it was quite impossible to 

 undertake the investigation of the negatives at the Lick 

 Observatory, owing to the limited stafi", and they were 

 therefore placed freely at the disposal of Prof. Weinek, 

 " whose previous experience in lunar observations and in 

 photography, as well as his very unusual artistic skill, 

 made his advice and assistance of extreme value." 



No pains have been spared to make the study of the 

 objects selected as complete as possible. As an instance 

 we may mention that Prof. Weinek's drawing of Coper- 

 nicus, enlarged twenty times from the negative, represents 

 the great labour of 224 ,' hours, and is described by Prof. 

 Holden as " a monument of skill and patience." 



It is proposed that a complete map of the moon, on a 

 scale of 3 feet to the diameter, shall eventually be made, 

 though the practicability of making a map on four times 

 the scale is demonstrated by an enlargement of Tycho. 

 The photograph of the Lunar Apennines, on the 3-foot 

 scale, reproduced in Fig. i., is a magnificent example of 

 a camera enlargement from one of the negatives. 



