September 6, 1894J 



NA TURE 



447 



approved Neo-Lamarckian manner the moulding of 

 limbs and the skeleton generally as the result of the 

 forces to which they are exposed during the life of the 

 individual. In the explanation of the remarkable 

 lengthening of the fore-part of the giraffe we meet with 

 a suggestion which is, I believe, new in the history of 

 I.amarckian speculation. In stretching for food it is 

 contended that the animal " must have constantly raised 

 itself otf its fore-feet, and as it dropped must have 

 received a shock that made itself felt from the hoofs 

 through the legs and vertical neck to the head. In the 

 hind-legs the shock would not be felt. . . . The principle of 

 increased growth in the direction of the shock resulting 

 from superabundant repair of the momentary com- 

 pression, explains how the giraffe acquired the pheno- 

 menal length of the bones of its fore-leg and neck ; and 

 the absence of the shock m the hind-quarters shows why 

 they remained undeveloped and absurdly dispropor- 

 tionate to the rest of the body." (pp. 164-165.) It is 

 unnecessary to take the trouble to refute the details of 

 the various suggestions brought forward by the upholders 

 of the Lamarckian hypothesis : they refute each other. 

 One of them explains a lengthened neck as the result of 

 extension, another as the result of compression ; while 

 neither give any approach to a proof that such an effect 

 is likely to result from the antagonistic forces which 

 they respectively invoke. E. B. P. 



CELESTIAL PHOTOGRAPHS. 



A Selection of Photographs of Stars, Star Clusters and 



Nebulcs, together with information concerning the 



Instruments and the Methods employed in the Pursuit 



of Celestial Photography. By Isaac Roberts, D.Sc, 



F.R.S., &c. (London : The Universal Press, 1894.) 



'T^HIS handsome collection of celestial photographs 



■^ is a remarkable example of what can be done 



single-handed in a new line of research. Taking up 



celestial photography more than ten years ago, Dr. 



Roberts has devoted himself entirely to it, and has been 



rewarded with an amount of success that must be to him 



a source of intense satisfaction. 



This success, however, has not been obtained by 

 merely exposing plates in the telescope and watching 

 the images come out in the developing dish. The work 

 that Dr. Roberts set himself to do was planned at the 

 outset in a thorough manner, and has been carried out 

 apparently without regard to either labour or expense ; 

 even to the extent of the removal of the observatory 

 near Liverpool, to a better position, selected after a most 

 exhaustive discussion of weather conditions in the south 

 of England. 



In the introduction of his book Dr. Roberts tells us 

 that a reflector of twenty inches aperture was the instru- 

 ment decided upon, in consultation with Sir Howard 

 Cirubb, as the most likely to give the best results. .In 

 instrument of this size, arranged for exposing the plate 

 without the use of a plane or prism, was made and 

 mounted by Sir Howard Grubb in 1885. With this 

 instrument all the celestial photographs have been taken. 

 The first arrangements do not appear to have been 

 NO. 1297, VOL. 50] 



quite satisfactory, so that the telescope had in great part 

 to be reconstructed ; but even then the star images were 

 found, on trial, to be elongated, though this defect seems 

 to have been remedied later on, if one may judge by the 

 remarkable absence of any deformation of image in 

 some of the very long exposure photographs. 



The question of the utility of the photographic chart 

 is discussed, and the fact mentioned that Dr. Roberts 

 had already, in 18S5, begun a photographic chart of the 

 northern heavens, and made some progress, when the 

 International Congress for the Photographic Chart of the 

 Heavens took up this work. The use of the paper copies 

 of celestial photographs might, it is suggested, be (l) 

 the detection of changes in the structure of nebula; (2) 

 the detection, on a large scale, of movements amongst 

 the stars ; (3) determinations of variations in stellar 

 magnitudes ; (4) relative distribution of stars in space ; 

 (5) detection of new stars and disappearance of others. 



In cases, however, where measurements to a second 

 of arc are required to be made, then the original 

 negatives must be used for the purpose ; and in this we 

 quite agree with Dr. Roberts. It would have been well 

 worth while to picture, in this connection, the instance 

 given by Dr. Roberts of his comparison of a chart of the 

 heavens, made in 1863 by D'.Vrrest, showing 212 stars, 

 with a photograph of the same region taken in 1S90, 

 which showed that considerable changes had taken place 

 among the stars in this small area of the sky (comprised 

 within one degree of declination and half a degree of 

 right ascension) during the interval of twenty-six years 

 between the epochs of the charts. 



The relative advantages of refractors and reflectors as 

 photo-instruments, and the requirements and adjustments 

 of a reflector for celestial photography are discussed ; and 

 the ideal instrument for photographing the sun, moon, and 

 planets is given as a refractor of six or eight inches 

 aperture and very great focal length ; but for the delinea- 

 tion of faint stars and faint nebulosity. Dr. Roberts 

 gives a preference to the silver-on-glass reflector, and he 

 would choose an instrument of twenty-seven inches clear 

 aperture and eleven feet three inches focal length, the 

 mirror being figured, and every part of the mechanism 

 made in the most perfect manner possible, and with a 

 guiding telescope with an objective ten inches in aperture. 

 To anyone contemplating the erection of an instrument 

 for celestial photography, these hints — the results of 

 many years' experience — should be of great value. 



The other chapters, on the collimation of the mirror, 

 the essentials of a photo-telescope, method of testing the 

 stability ofa photo-instrument, and photographic plates, 

 their exposures and developments, will be read with great 

 interest by all photo-astronomers. 



The chapter on the collimation of the mirror gives an 

 account of the method employed to check the guiding 

 telescope, and is perhaps the only point on which some- 

 thing might be said in the way of criticism of Dr. 

 Roberts' method. It is difficult to see why the method 

 described should not be varied so as to be; always avail- 

 able, and so dispense with the guiding telescope ; a matter 

 of no slight importance in the case of such an instrument 

 as Dr. Roberts has suggested, the ten-inch objective of 

 which would cost as much or more than the mirror. We 



