282 



KNOWLEDGE 



[December 2, 1895. 



there will be little risk of error in recognizing it on care- 

 fully exposed and carefully measured negatives. This 

 process of elimination may go very far, leaving a physical 

 group scant in number comparatively to the optical group 

 with which we are at present acquainted. Those among 

 us will know the npshot who live to see the next return of 

 Halley's comet. 



One element of the motion of Alcyone and its com- 

 panions might indeed befoi-e now have been ascertained ; 

 and, considering the special interest of the subject, it is 

 rather surprising that the spectrographic method has not 

 yet been appUed to its investigation. Line-displacements, 

 produced by the retreat from the cluster of the solar 

 system, should at any rate be perceptible in the spectra of its 

 components ; and individual modifications of this general 

 effect might also be brought to light. The research is one 

 of great promise. Data derived from it, if of the order of 

 accuracy now easily attainable, will be of crucial im- 

 portance ; and they will gratify, not prospective, but 

 immediate curiosity. They will not be garnered only for 

 the benefit of posterity. Let us hope that they may be 

 promptly obtained. 



ON THE EXTERIOR NEBULOSITIES OF THE 

 PLEIADES. 



By Prof. E. E. Barnard. 



IN the AstronviJiischf XKcJirichtin, No. H253, 1 have given 

 an account of a long-exposure photograph of the 

 Pleiades. A diagram drawing accompanies the 

 article, which shows the positions of a number of faint 

 nebulous masses and strips extending in all direc- 

 tions about the Pleiades, and apparently connected with 

 the nebulosities of the cluster itself. 



This picture was made with the six-inch WiUard lens of 

 the Lick Observatory on the nights of December Gth and 

 8th (the 7th was cloudy), 1893, the exposure being 

 lOh. 15m. 



It was my intention to repeat this exposure in the past 

 winter with a greater duration, but the winter proved the 

 most unfortimate for observation that we have yet liad at 

 Mount Hamilton, and nothing could be done while the 

 Pleiades were in a favourable position. 



I have, however, succeeded in making an enlargement 

 from the negative of 1893, that shows the principal ones 

 of these nebulosities quite nicely. 



Some of the fainter streams and masses are lost in this 

 enlargement, but enough remains to show clearly the 

 nature of the nebulosities and their relation to the Pleiades. 



Two irregular streams of nebulosity are shown in the 

 picture, running towards the east. They flow from the 

 north and south sides of the cluster, and run brokenly 

 for three or four degrees easterly. These are the most 

 prominent of the new nebulosities. 



Looking carefully at the picture, it will be seen that the 

 northern stream is double for a good part of its length, the 

 northern portion being very faint. 



To show these faint nebulosities it was necessary to 

 sacrifice all the nebulosities of the immediate group along 

 with the bright stars of the cluster. 



This is of no importance, however, since they are all 

 well known through previous photographs. The present 

 picture is intended only to show the new nebulosities. 



On this subject I have an interesting letter from Dr. H. C. 

 Wilson, of theGoodsell Observatory, Northfield, Minnesota. 



Speaking of these distant nebulosities about the Pleiades, 

 Dr. Wilson says, .June 2Cth, 1895 : 



" I have an eleven hour exposure on the Pleiades region 



taken on October 23rd, 24th, 26th, 1894, which shows 

 most of the great nebula sketched by you from your photo- 

 grajihs. It is so delicate that I did not see it until I made 

 a positive which considerably increased the contrasts." 



The small illustration is part of a duplicate negative of 

 the Pleiades region.* This gives a good idea of the arrange- 

 ment of the stars in that region of the sky, and their rela- 

 tion to the Pleiades. 



This picture was made 1893, December 1st, and was 

 given four hours exposure (from C.35 p.m. to 10.35 p.m.). 

 In the north-west part of the entire plate a meteor trail is 

 seen, directed nearly towards the Pleiades, but it does not 

 extend into the part of the plate reproduced. 



SPECTRUM ANALYSIS.-III. 



By J. .T. Stewabt, B.A.Cantab., B.Sc.Lond. 



THE application of the spectroscope to the determina- 

 tion of tlic chemical nature of a substance depends 

 upon the peculiar nature of the spectrum given by 

 incandescent gases, which was referred to in a pre- 

 vious article (Knowledge, No. 117, p. 150). This 

 consists of one or more bright lines of definite colour sepa- 

 rated by dark spaces, and the number and position of these 

 lines is characteristic of the gas which gives out the light. 

 Each element, when in the state of a glowing gas, possesses 

 a spectrum which is peculiar to itself and differs from that 

 of all other substances. The position of the bright 

 lines in the spectrum of a given substance can be noted, 

 and if bright lines occupying these positions occur in the 

 spectrum of some compound which is under investigation, 

 the observer knows that the element in question must be 

 present in the compound. When the temperature of the 

 fiame is high the compound is probably dissociated or split 

 up into its constituent elements, each of which gives its 

 own spectrum with its characteristic lines indicating clearly 

 the presence of the particular element. 



That the presence of these lines shows distinctly that a 

 certain element must be present follows from the fact that 



A B C D = Openings in a G-rating ; L = lens ; F = focus. For full 

 explanation of the (Uiigvam xee Knowiedge, November, 1895. p. 250. 



in no case has it yet been clearly shown that any of the 

 lines of two elements coincide in position. They do not 

 overlap, and when two lines in two difi'erent spectra 

 occupy the same position, this indicates that there must 

 be some element which is common to the two substances 



* Tliis plate has been accictcntally reversed, right for left, in 

 eopving. This smaller photograph coTers very nearly the same area 

 as tile larger. The scale of the larger is Imm to 2' of arc; of the 

 smaller 1mm to -i'^ of arc. 



