100 



KNOWLEDGE. 



[April 1, 1897. 



MIRA CETI. 

 To the Editors of Knowledge. 



Sirs, — Owing to the long-continued spell of weather 

 unfavourable to astronomical work, my opportunities for 

 observing Mira have been very few at this apparition. 



The star was plainly visible to the naked eye on 

 November 2'.)th, and has been looked for on every evening 

 since that date when there was the slightest chance of its 

 being seen. It has, however, not been caught sight of for 

 the past six weeks, and will now set too soon after the 

 sun. 



According to the observations given below, a maximum 

 occurred on January 23rd or 21th. 



PHOTOGRAPH OF THE GREAT NEBULA 

 IN ORION. 



By Is.4AC Egberts, D.Sc, F.E.S. 



THE photograph of the great nebula in Orion, an 

 enlargement of which to the scale of one millimetre 

 to twenty seconds of arc is annexed, was taken 

 with the twenty-inch reflector ; the negative had 

 a dual exposure, with an interval of five days 

 between them — first on the 3rd and the second on tlie 

 8fch I'ebruary, liS94 — the full exposure being seven hours 

 and thirty-five minutes, and the photograph depicts, very 

 probably, the maximum of extent and detail that can be 

 shown by aid of photographic methods. 



This statement is of great importance iu astronomical 

 photography, for, if correct, it proves that, as regards dura- 

 tion of exposure, wc have already reached the limit of the 

 available aid by this method in astronomical researches. 

 It is therefore essential that I should give my reasons for 

 arriving at this inference ; they are the following : — (1) The 

 film of the negative is, in consequence of prolonged ex- 

 posure to the latent sky luminosity, darkened on develop- 

 ment to a degree that would obscure faint nebulosity and 

 faint stars. (2) Longer exposures of the plates would not 

 reveal additional details of nebulosity nor more faint star 

 images. 



I am fully aware that doubts as to the accuracy of these 

 statements will be raised, and I will, in anticipation, here 

 refer to some of them, (a) An exposure of twenty-four 

 hours fifty-three minutes has been given to a plate upon 

 the region of Eta Argus by Dr. Gill at the Cape Observa- 

 tory without the film being too densely darkened, and I am 

 under the impression that Dr. Max Wolf and others have 

 given long exposures to plates in portrait-lens cameras, 

 but 1 have not seen any reports concerning the results 

 bearing upon this question. (//) The plates I have used 

 may not have been sufficiently protected from light, (o) 

 The nights on which the photographs were taken may not 

 have been normally clear. (</) Extraneous light may have 

 somehow affected the film. To all these and to other 

 suggestions of a similar character, I would answer that 

 due and proper care has been exercised in guarding against 

 the occurrence of these contingencies ; and that the darken- 



ing of the sensitive films, by long exposure to the sky in 

 the twenty-inch reflector, is proportional to the duration 

 of the exposure and the quality of the night — the longer 

 the exposure the darker the film becomes in the develop- 

 ment of the images. The sequence here stated has been 

 observed during many years on all very sensitive films 

 which have had long exposure, and found to be practically 

 invariable ; at the same time the unexposed margins of the 

 films are nut darkened, hut irmain clear. 



The reason why a refractor of foeal distance to aperture 

 in the ratio of one to ten could not cause the darkening ot 

 the film, nor depict faint nebulosity and faint stars, so 

 rapidly as a reflector of the focal ratio of one to five, is 

 consequent upon the greater power of the latter in the 

 concentration of light upon the sensitive film than is 

 possessed by the refractor ; there is also loss of light by 

 reflection from the surfaces of the lenses, by absorption, 

 and by absence of perfect achromatism in the lenses. 

 Less sensitive plates may also have been used in the re- 

 fractor than were used in the reflector, and the develop- 

 ment of the plates may not have been carried to the same 

 degree of intensity. Therefore on consideration of all the 

 relative conditions, twenty-five hours' exposure of a plate 

 in the refractor may not be so near the limit of practicable 

 photographic efi'ect as seven and a half hours would be 

 with the twenty-inch reflector. 



Many photographs of the nebula here referred to have 

 been taken since the year 1880, when the late Dr. Henry 

 Draper, in America, took the first ; and it is regretable 

 that scant justice has been done to his memory as the 

 pioneer of nebular photography. Those who have referred 

 to his work do not seem to have noticed the reproduction 

 of his photograph of the nebula in Orion in Prof. 

 Holden's monograph, published at Washington in 1882, 

 PI. XL., pp. 226-230. This photograph ought in fairness to 

 be designated the premier one, for there is very little 

 difference between it and that taken by Dr. Common in 

 1883, and I do not think that Dr. Common himself has 

 ever put forth a claim for great advancement in his photo- 

 graph beyond that by Dr. Draper, taken in March, 1882, 

 and referred to above. 



The negative and the positive copy of it on glass, more 

 than the photograph hereto annexed, depict the nebulosity 

 in marvellous detail, and in curdling visible forms — quite 

 indescribable by language — from the extreme boundaries 

 of the cloudlike matter to the dense centre around the 

 trapezium ; they also show the structure and extent of 

 the northern part of the nebula (h 1180), together with 

 the vortical faint cloudiness which extends from this part 

 of the nebula to the other, and connects them together ; 

 thus pointing to the common origin of the three nebube, 

 M 42, 43, and h 1180. 



The southern margin of the nebula extends to the star 

 Eta, and is lost in the atmospheric glare that appears like 

 nebulosity surrounding that star. 



It would not be profitable at the present time to enter 

 into speculative matters concerning the forces that may 

 have originated this nebula, but it is evident on close ex- 

 amination of the negatives — of which I now have several, 

 taken during the past eleven years — that there are vortical 

 areas within the nebula, and that they curve and trend in 

 various directions : they resemble eddies in whirlpools, or 

 puffs of steam ejected into the atmosphere. There are 

 also several obviously variable stars in different parts of 

 the nebula, but I have not been able to detect with 

 certainty any striking or readily seen change in the 

 structure of the nebulosity shown on the photographs 

 which have been taken during the past seven years. This 

 would indicate tliat the nebula is at a great distance from 



