82 



SCIENCE. 



not more than 3 or 4 . In fact, about four o'clock the 

 transmitted light was of a splendid green color, tinting the 

 white walls of my room as though through the stained 

 glass of a church. About the time I first noted the 

 colors a strong north wind sprung up, continuing in gusts 

 through the afternoon. F. H. Lond. 



Colorado Springs, January 29, 1881. 



NOTE ON DR. HENRY DRAPER'S PHOTO- 

 GRAPH OF THE NEBULA IN ORION* 

 By Mr. Ranyard. 



*Read before the Royal Astronomical Society, Jan. 14, 1881. 



Dr. Draper has sent me an enlarged copy of a photo- 

 graph of the nebula in Orion, which he succeeded in tak- 

 ing on the night of the 30th of September last. Dr. 

 Draper remarks that September is not the best time of 

 the year, so that he hopes to obtain still better results 

 next summer. The photograph was taken with an ex- 

 posure of 51 minutes. He does not mention the instru- 

 ment with which it was taken, but I conclude that it was 

 with his great 27-inch reflector. On the photograph are 

 nine white spots of various sizes; these represent 13 stars 

 in and about the nebula, for the four stars of the trape- 

 zium are merged together by reason of over-exposure. 

 In the corner is another small photograph taken with a 

 shorter exposure, and showing three of the four stars of 

 the trapezium. This is not the first occasion on which 

 the stars of the trapezium have been photographed. I, 

 and no doubt many others, have succeeded in obtaining 

 photographs of them. But it is, I believe, the first pho- 

 tograph in which any trace of the nebula is shown. And 

 Dr. Draper may, I think, be very much congratulated on 

 the great success he has attained. The photograph 

 shows the whole of the brighter nucleus of the nebula — 

 sometimes referred to as the " Fish's head." I have com- 

 pared it with the different drawings of the nebula by 

 Bond, Herschel, L'.aponnov, Lassell, Secchi, the Earl of 

 Rosse, and Tempel, and find that it does not correspond 

 exactly with any of them. The drawings differ very 

 greatly amongst themselves, and they differ in type as 

 well as in minor details. They do not appear to differ 

 continuously in order of time, so that the drawings do 

 not afford any proof that the form of the nebula is chang- 

 ing. Photographs will of course afford much more valu- 

 able evidence with respect to any such change in the fu- 

 ture. The photograph does not show any stars of less 

 than the g]4 magnitude, showing that the brighter 

 masses of the nebula registered themselves on the plate 

 when stars of the ioth magnitude left no trace. If in 

 the future some much more sensitive method of photo- 

 graphing is devised, it will be necessary to contrive some 

 plan by which the brighter parts of the nebula and the 

 light of the brighter stars may be cut off from the sen- 

 sitive plate during the greater part of the exposure, 

 so as to prevent the irradiation from the brighter parts 

 encroaching over the area occupied by the fainter parts. 

 At present, however, we are very far from being able to 

 photograph, with the sensitive silver compounds* made 

 use of, all that can be seen with the human eye. But 

 even if photography does not make any further advances, 

 photographs such as these will be of very great value in 

 showing the relative brightness of the brighter parts of 

 the nebula. 



Mr. Common : I do not agree with Mr. Ranyard, that 

 we must look to photography to explain or prove any 



♦[Note by Mr. Kanyard.j It seems probable thit the small pencil of 

 light, which passes through the pupil of the eye from the faintest object 

 perceived, produces an actual change in the matter of the rods and cones, 

 which is rapidly obliterated by the circulation and vital processes going 

 on about the retina. This is now, 1 believe, pretty generally agreed to 

 by physiologists. If in the future the matter acted upon in the rods and 

 cones can be isolated, and the change produced by light can be rendered 

 permanent, it seems probable that, by means of large lenses and reflectors, 

 we may some day obtain photographs of objects too faint to be visible with 

 the naked eye. 



change in the form of the nebula?, because various kinds 

 of plates give different results, and you would not have 

 the same effects produced by the same colored light. I 

 should rely much more on accurate drawings than upon 

 any photographs. If we compare these drawings, here 

 you have [pointing to Father Secchi's drawing] a dark 

 mass with a slope of light running from the left-hand 

 corner down to the right hand. In the other [Lord 

 Rosse's drawing] there is no division, except a large 

 space divided into channels. The latter is wrong and 

 the former clearly right. Before you give details you 

 ought to represent the chief features of the nebula, be- 

 cause it is the features that most readily indicate change. 

 With regard to Mr. Ranyard's remark that no star 

 smaller than the ioth magnitude is shown, there are, I 

 think, two — these fainter stars under the trapezium, 

 which are certainly less than the ioth magnitude. 



Mr. Ranyard : I have here the magnitudes given by 

 Liaponnov, and he gives one as the 9th magnitude and 

 the other as the 9th to the ioth magnitude. 



Mr. Common : Before we can discuss this photograph 

 we want to know the instrument it is taken with, the focal 

 length, in order to know the size of the image, and the 

 kind of plates used, and the mode of development. If 

 you want to detect any change in the form of the nebulae 

 you must entirely rely on the hand drawings. 



Mr. Ranyard: I think that some considerable scientific 

 use may be made of these photographs ; they will at least 

 enable us to compare the relative brightness of the differ- 

 ent masses of the nebula as shown on any one photo- 

 graph, for as far as we know, there is no great difference 

 in the spectrum of different parts of the nebula, and 

 so we have no reason to suppose that the photographic 

 effects of different parts of the nebula in any one photo- 

 graph would not be proportional to the light. 



Mr. Stone : With regard to discrepancies in drawings, 

 I never knew two persons asked to make a drawing 

 of the same faint object make them exactly alike. It 

 is evident that observers draw that which happens to 

 arrest their attention, and one feature will strike one 

 observer, while the attention of another is attracted by 

 something else. A very good instance of this occurred 

 during the eclipse of 1874. Two observers were sitting 

 side by side drawing the corona. The one drew a small 

 nearly quadrilateral corona, while the other drew a large 

 corona with great rays in the equatorial regions. Before 

 a totality was over the observer who had drawn the small 

 corona looked at his neighbor's drawing, and, on looking 

 up again at the corona, recognized the outline which his 

 neighbor had drawn, and commenced to put it on paper 

 when the eclipse ended. There is therefore a great ele- 

 ment of uncertainty about drawings, one observer over- 

 looks one part, or is struck by one part, and another by 

 something else. 



Mr. Rand Capron : I think that Mr. Commom is right, 

 that photographs of objects taken with different instru- 

 ments and plates will probably never usefully bear com- 

 parison ; but I agree with Mr. Ranyard that photographs 

 of the same object taken from time to time with the same 

 instrument and the same plates can most usefully be 

 compared. 



Mr. Burton said : I should like to suggest that the diffi- 

 culty which Mr. Ranyard has referred to, with regard to 

 the irradiation from stars interfering with the fainter 

 parts of the nebula, might begot over by placing a prism 

 of small angle, made of quartz or Iceland spar, between 

 the object-glass and the photographic plate. The images 

 of the stars would be drawn out into lines, while there 

 would be three or four images of the nebula which would 

 not interfere. The principal plane of the prism mi^ht 

 then be turned round into a different position-angle, and 

 another photograph taken, so that the spectra of the stars 

 would fall in another direction. 



Mr. De La Rue said : I recollect very well the time 

 when the Earl of Rosse's drawing was made. I compared 



