December 2, 1889.] 



KNOWLEDGE 



35 



not to do an injustice to anyone. The son to whom he 

 refers received last year a powerful electric shock through 

 his body from hand to hand, from wliicli he still lies an 

 almost helpless wreck, unable to rise, and hardly able to 

 lift his arms. This is the illness to which the letter refers. 

 We are now so widely welcoming the Dangerous Demon of 

 Electricity, that the accident to this promising young 

 electrician (whom I remember as an Eclipse observer in 

 Colorado in 1878) cannot be too widely used as a warning. 

 [Our last number was not a fortunate one in which to 

 alliule to the mistakes of American printers. It contained 

 a good many printers' errors — probably due to the tact 

 that it was the lirst number printed by fresh printers. — 

 A. C. Eanyard.] 



COMPARISON OF PHOTOGRAPHIC EFFICIENCY OF 

 TELESCOPES. 



To the Editor of Knowledge. 



Dear Sir, — Visually, I presume there is no difference of 

 opinion that the faintness of a star which may be seen 

 with a telescope depends almost entirely on the square of 

 the diameter of the lens, and is independent of the focus. 

 If the seeing is very bad, of course the large glass labours 

 under a slight disadvantage ; but we are assuming reason- 

 ably good astronomical conditions. In looking for faint 

 stars, we would naturally use as low a power as the tele- 

 scope would bear. 



Photographically, diffraction has very little to do with 

 determining the size of the image of a star. The size 

 depends chiefly on the steadiness of the air, the accuracy 

 of the following, and the correctness of the lens. The 

 formuliE expressing it must therefore be only approximate 

 and entirely empirical. It has been found that mider 

 favourable conditions at Cambridge with a 3 -inch 

 camera lens, the images of the fainter stars were -035 mm. 

 in diameter. With an 8-ineh camera they were -028 mm., 

 and with a 13-inch telescope -047 mm. The two latter 

 instruments were finished by the Clarks. If the same had 

 been true of the former, the images would undoubtedly 

 have been smaller. 



If faint star images were always of the same size, the 

 faintness of a star which we might detect by photography 

 would be inversely proportional to the square of the aper- 

 ture of the instrument and mdependent of its focus — the 

 same as for visual telescopes. As stated in my former 

 communication (in connection with the formuhe then 

 given), this is perhaps hardly fair to the larger instrument, 

 as an increase of focus gives slightly larger images, and a 

 modification in the formula' is therefore introduced. 



In Canibridgc, under favourable circumstances, with the 

 13-incb lens, the diameter of w photograpliic star image is 

 about 2". 



As regards the Orion nebula, 1 Imve l)een more delayed 

 "lliiui I liad expected, as I find on close examination that 

 oui- negatives show much more detail tlian I bad at first 

 supposed. This involves a good deal more work, but I am 

 hoping to have my results in final shape in the course of a 

 few weeks more. 



^'ery truly yoiu-s, 



^\'. 11. PlCKKRINO. 



Harvard College Observatory, Cambridge, U.S.A. 



[The diameter of 2" mentioned by Prof. W. H. Picker- 

 ing is very small, and would seem to show that at Harvard 

 the atmosphere is occasionally very tranquil. With two 

 instruments of equal diameter, one double the focal length 

 of the other, we should expect the same atmospheric in- 

 equalities to cause double the swing in the longer focussed 

 instrument, that is, the luminous image of a star would 



travel over an area four times as great on a photographic 

 plate in the focus of the longer instrument than on one in 

 the focus of the shorter instrument. But the disc and 

 rings are of double the diameter, and, area for area, of one 

 fourth the intensity in the longer focussed instrument. 

 The actual dilferenee in the diameter of the initial patch 

 of photographic action needs to be determined by experi- 

 ment, before photographs taken with different instruments 

 can be used for comparing the tranquillity of the atmo- 

 sphere at two stations. — A. C. E.] 



To the Editor of Knowledge. 



Dear Sir, — A fireball of remarkable brilliancy was 

 observed here on November 4th, at 7h. 55m. G.M.T. It 

 appeared to the north and below the pole star. That por- 

 tion of the heavens was considerably obscured by cumulus 

 clouds, and the passage of the fireball was shown by a 

 series of flashes as it passed the intervening clearer spaces, 

 which might have been taken for flashes of lightning. So 

 far as could be made out, a line drawn from (3 Praconis to 

 Castor would roughly indicate its path, and it moved from 

 a considerable altitude obliquely downwards toward the 

 N.E. horizon. 



When first seen it equalled Venus at greatest brilliancy, 

 and rapidly increased in size and brightness. AVhen 

 below the pole it burst with an intensely brilliant flash, 

 which illuminated the whole northern region, but instead 

 of being dissipated as usual, a large portion, fully equal 

 to .Jupiter in brightness, and much brighter than a first 

 magnitude star, continued its course towards the N.E. 

 horizon. The moon, about eleven days old, was shining 

 brilliantly at the time. The duration was not more than 

 six or seven seconds, and the light was decidedly blue. 



See also Xntwv, Nov. 14, p. 32 ; also Xature, Nov. 21, 

 p. 60, which must all refer to the same, and may interest 

 you. — Yours truly, 



C. E. Peek. 



Eousdon Observatorv, Lvme. 



'I'll the l-'.ditor of KxOWLEDGE. 



Sir, — Possil)ly the following may now be of some 

 interest. 



Some years ago I was in a small brick house in the Kew 

 Eoad, Eichmond. In a room on the ground floor an ordi- 

 narv " register stove " fire-grate, filled witli coal, was 

 covered from the back to the hearth with a piece of news- 

 paper ; over the lower two-thirds of this was placed a 

 sheet of cut tissue paper, of a bright green colour, and on 

 the upper two-thirds, over all, was a sheet of pink tissue 

 paper. Thus about one-half of the green paper was 

 placed between one-half of the pink paper and the news- 

 paper, and in contact with both. 



One morning I found that in the night the whole of the 

 green paper, except a few scraps of the corners, had been 

 reduced to ash ; but the pink paper and the newspaper 

 were uninjured, with the exception of a few small burnt 

 holes. 



The grate was in an outside wall, and on the outside 

 of the wall was an iron rainwater pipe from the roof, cut 

 ofl' a foot or more i'roin the ground. 



I was careful to aseertain at the time that no one in the 

 house sould by accident or otherwise have done it. Any 

 kind of trick was quite unimaginable. 



I can only suppose that the cause was a slight electric 

 discharge. 



Yours faithfully, 



128, Mount Street, W. T. S. Petty. 



