NATURE 



533 



THURSDAY, APRIL 5, 1900. 



CELESTIAL PHOTOGRAPHS. 



9tographs of Stars, Siar-cltisters and Nebulae, to- 

 tther with Records of Results obtained in the pursuit 

 Celestial Photos^raphy. By Isaac Roberts, D.Sc, 

 .R.S. Vol. ii. Pp. 178. Plates, {l^ondon : Know- 

 edge Office, 1900.) 

 is now nearly six years ago since Dr. Roberts pub- 

 lished his first volume on celestial photographs, 

 :h was noticed at some length in these columns 



^)1. I. p. 447). It was there remarked that the volume 

 was the result of a " remarkable example of what can be 

 done single-handed in a new line of research," and we 

 might echo the same statement as regards the contents 

 of the present issue. 



Every astronomical reader is familiar with the first 

 publication ; indeed, Dr. Roberts's celestial photographs 

 of long exposure were, and still are, so remarkable that 

 many of them have been reproduced in most of the 

 more recent works on astronomy. It is interesting to 

 remark that in commencing astronomical photography it 

 was the author's original intention to make a photographic 

 chart of the sky between the north pole and the equator, 

 so that those who came after him could, by taking similar 

 photographs and comparing them with his, detect any 

 changes that might have taken place during the interval 

 that had elapsed. After he had secured many photo- 

 graphs on a definite programme of work, the international 

 scheme for making a photographic chart of the whole 

 heavens was suggested and commenced under the direc- 

 tion of the late Admiral Mouchez. Dr. Roberts therefore 

 discontinued his charting work, and began the important 

 investigation of photographing, on a large scale and with 

 long exposures, the various star-clusters and nebulae with 

 the object of securing exact pictures of them, so that any 

 changes that might take place in them might be detected 

 after the lapse of some years. 



The first volume indicated to the astronomical world 

 the great and well deserved success which rewarded the 

 labours of Dr. Roberts in this, perhaps, the most interest- 

 ing branch of astronomy, and he may be said to have 

 continued with the photographic plate the work that the 

 Herschels accomplished visually with their giant tele- 

 scopes. Like these celebrated observers, he has photo- 

 graphed " double and treble nebuUe variously arranged : 

 large ones, with small, seeming attendants ; narrow, but 

 much extended, lucid nebulte and bright dashes ; some of 

 the shape of a fan, resembling an electric brush, issuing 

 from a lucid point ; others of a cometic shape, with a 

 seeming nucleus in the centre, or like cloudy stars sur- 

 rounded with a nebulous atmosphere ; a different sort, 

 again, contains a nebulosity of a different kind, . . . .; while 

 others shine with a fainter, mottled kind of light, which 

 denotes their being resolvable into stars." 



Of the seventy- two objects enlarged from the original 

 negatives, and here beautifully reproduced in collotype 

 by the London Steroscopic Company, thirty-three are of 

 spiral nebuUe, fifteen of clusters, fourteen of nebuhe, 

 irregular and cloudlike in form, six of crowded star areas, 

 NO. 1588, VOL. 61] 



and four of annular nebula^. The original photographs, 

 which are 15 centimetres square,^ were all obtained, as 

 formerly, with the silver on glass reflector of 20 inches 

 aperture and 98 inches focal length. It may be here 

 mentioned that Dr. Roberts has added to his instrumental 

 equipment a specially made Cooke triplet portrait lens 

 of 5 inches aperture and I9"22 inches focal length, with a 

 photographic field of i 5 degrees diameter. 



In the work before us, the arrangement of the plates 

 differs from that adopted in the first volume. The photo- 

 graphs, instead of following each other in the order of 

 right ascension, are here divided into classes or groups, 

 each of which indicates apparent physical relationships, 

 and the members of each group are arranged as far as 

 practicable in the order of right ascension. The scale 

 of enlargement is given in each case, as well as a table 

 for converting the measured right ascensions of the stars 

 shown on the photographs into intervals of time for each 

 degree in decimation. It may be remarked that the 

 table of corrections to be applied to the scales of the 

 photo-plates which appeared in the first volume has 

 been dispensed with, owing no doubt to the improvement 

 in the manufacture of photographic films. The co- 

 ordinates of each of the fiducial stars are given for the 

 epoch 1900, and on the plates these stars are marked 

 with dots as formerly. 



In the reproduction of such difficult objects as those 

 here illustrated, it is well known that much fine detail is 

 lost in the process. Reproductions, although approxi- 

 mating closely, yet never come up to the quality of the 

 original negatives. The last mentioned, however, are 

 subject to many vicissitudes. They can become broken, 

 the films become discoloured after some time, images 

 fade, and faint nebulosities disappear entirely. That 

 such is the case is clearly proved by the experience of 

 Dr. Roberts, which is related in his introduction. 



To mention only one instance of many, he tells us 

 how, shortly after a photograph of a certain region of the 

 sky was taken (in February 1886), he counted 403 star 

 images on the negative. On May 29, 1895, oi" after an 

 interval of nine and a quarter years, no less than 131 

 stars had disappeared from the same plate, he being only 

 able to count 272 images. 



With such facts before us, it is therefore of great 

 importance that as each negative is secured an impres- 

 sion of it should be made in permanent form, such as in 

 printer's ink. If the work be done well, as is the case 

 with the beautiful illustrations in this volume, future 

 astronomers will have valuable data at their disposal for 

 making direct comparisons. 



In the introduction. Dr. Roberts refers to several 

 points of great interest, which will be read generally 

 with advantage, but especially by those who expose 

 their plates to the sky for long intervals of time. He 

 first gives us an account of his experiments regarding 

 the effect of " atmospheric glare," which is due to star- 

 light, causing a general fogging of the whole photo- 

 graphic plate. For exposures extending over several 

 hours, Dr. Roberts is led to deduce that, at any rate for 

 this country at least, by the time that the image of an 

 eighteenth magnitude star is well impressed on the 

 photographic plate, the whole plate has become so 



A A 



