448 



NA TURE 



[September 6, 1S94 



believe Sir Howard Grubb has already suggested some 

 such method. 



We should have very much liked further information 

 about the instrument, the supports of the mirror and the 

 plate-holder, and other details, but these are perhaps 

 rather in the province of the telescope-maker than the 

 photo-astronomer. .A. photograph of the double tele- 

 scope, however, gives a very good idea of the general 

 arrangements, and there is also a photograph of the 

 observatory. 



Of the forty-nine celestial objects reproduced on paper, 

 four are star charts, twenty-two are clusters of stars, and 

 twenty-three are nebula;. The earlier and most im- 

 portant photographs in this collection have already been 

 published in various ways, and many of the others 

 exhibited at the meetings of the Royal .-Xstronomical 

 Society ; for Dr. Roberts, with proper scientific spirit, 

 has always been ready to place any of them at the 

 disposal of astronomers : but they lose nothing of their 

 great interest by being seen in the collected form. 



Had Dr. Roberts' work been only to produce the first 

 photograph, that of the great nebula in .Andromeda, and 

 had that been the only result of all his labour, he would 

 have been amply repaid, for it is certainly not too much 

 to say that, in this picture, photography has done more to 

 justify its use by the astronomer than in any other case ; 

 and one can feel that the hopes that were formed by 

 Rutherford and others in the early days of photography, 

 and which lay dormant for so many years, have now 

 been realised. The perfection of delineation enables 

 the true character of this marvellous object, which 

 remained hidden from the closest scrutiny of such a 

 careful and competent investigator as G. P. Bond, to be 

 at once seen and appreciated. It is difficult to imagine 

 that such an enormous object as the Andromeda nebula 

 must be, is not very near to us ; perhaps it may be found 

 to" be the nearest celestial object of all beyond the Solar 

 System. It is one that offers the best chance of the 

 detection of parallax, as it seems to be projected on a 

 crowd of stars, and there are well-defined points that 

 might be taken as fiducial points for measurement. 

 Apart from the great promise this nebula thus seems to 

 give of determining parallax, there is a fair presump- 

 tion that in course of time the rotation of the outer j 

 portion may perhaps be detected by observation of the 

 positions o( the two outer detached portions in relation 

 to the neighbouring stars. 



The photographs are arranged in order of right ascen- 

 sion, each has its position given, and many have marked 

 fiducial stars whose position is given for 1900. The 

 scale of enlargement is also given, so that with the help 

 of a table of corrections of errors due to the slight altera- 

 tions in scale in enlarging or printing, any point on 

 the photograph can be determined with an accuracy 

 sufficient at least for purposes of identification. 



For some purposes, no doubt, it will be absolutely 

 essential to have recourse to the original negatives, but, 

 as Dr. Roberts has pointed out, there are many purposes 

 for which these paper copies are available. In addition 

 to the full date of each object, which is admirably done, 

 there are various useful references to previous observa- 

 tions. The accuracy with which the telescope is kept 



NO. I 297. VOL. 50] 



pointed during a long exposure is seen by a glance at the 

 shape of the brighter star images, as a shift, if not at once 

 corrected, produces deformation. In nearly every photo- 

 graph the stars are round, in some they seem absolutely 

 so, which shows the perfection of the instrumental 

 arrangements of Sir Howard Grubb, and the care with 

 which the exposures have been made. Some of the star 

 photographs — as, for instance, plate 43 — show a perfection : 

 of image and an absence of distortion over a large field ] 

 that is very satisfactory. A comparison with a plate of J 

 the same region taken by an International Chart refractor 

 would be interesting. 



In looking at the examples of spiral nebuhc, of which 

 there are several, one cannot help thinking of Lord 

 Rosse, and the correctness with which he delineated 

 these objects, though his views at the time were not 

 generally accepted by astronomers. These most in- 

 teresting nebula." will repay further study, perhaps, more 

 than any other. 



Some of the cluster photographs are too much en- 

 larged for the ordinary distance at which one reads a 

 book, but the proper effect can be obtained by looking at 

 them about three feet away. .All the photographs are 

 about as good as it is possible to take with an instru- 

 ment of the size used by Dr. Roberts, and the present dry 

 plates ; better photographs can only be obtained by the 

 use of a larger telescope, or by means of dry plates that 

 will, with the same or greater sensitiveness, give a finer 

 grain. This is shown by a comparison of the photograph 

 of the cluster .M. 5, taken by Dr. Roberts, with one taken 

 with a larger telescope of a similar kind. Dr. Roberts 

 states (p. 91) that "the cluster is enclosed in dense 

 nebulosity about the centre, the nebulosity hides the 

 stars, even on the negative"; but an inspection of the 

 photographs by a larger instrument shows that the stars 

 are quite distinct, though the exposure was very much 

 longer, a result that might fairly be expected. 



A comparison of photographs of this object, taken at 

 different dates, shows that many of the stars of this cluster 

 are variables, and of course it is just possible that there 

 may be variability in the central portions. 



Excellent as are the paper reproductions of these photo- 

 graphs, they cannot give all the beautiful detail to be 

 seen on the original negative of a nebula. If change be 

 taking place, it is more than likely that it may be seen in 

 the finer detail first, in which case the original negative 

 must be seen. This points to the absolute necessity of 

 preserving the negative where it shall be available in 

 after years. We hope that all valuable originals willfind 

 their way eventually to the keeping of the Royal .Astro- 

 nomical Society, which now undertakes this important 

 duty. 



This volume shows that Dr. Roberts has spared neither 

 himself nor his purse in fulfilling the task he set himself to 

 do, more than ten years ago. By the great labour and 

 devotion he has shown for many years in carrying out 

 his carefully considered plans, he has set a model for 

 others to follow in taking up a research of this kind, of .i 

 which there are so many in astronomy, that from the |y ' 

 nature of the case can only be well done by those wln', 

 like him, undertake it solely as a labour of love. 



A. A. Co.MMO.v. 



