September 1, 1890.] 



KNOWLEDGE 



213 



as it registers the appearance of a region poor in lines. 

 There are three or four narrow spaces in this region of the 

 spectrum which look very like bright lines, and it is diffi- 

 cult to convince oneself that they are not brighter than 

 the rest of the backgi-ound of the spectrum. I am in- 

 clined to agree with Dr. Draper that they are actually 

 brighter spaces in the solar spectrum. There are similar 

 bright spaces in the portion of the spectrum shown in 

 Section 7, near to wave-lengths 3884 and 3888. 



The nebulous K and H lines, shown in Section 6, are 

 seen to contain a great number of fine lines which are, 

 however, not symmetrically arranged with respect to the 

 centres of the nebulous bands, and therefore probably 

 cannot be regarded as forming a group associated witli the 

 hazy lines on which they happen to lie. 



The concave grating used by Mr. Higgs is mounted on 

 one side of a circular table, ten feet two inches in dia- 

 meter. The sensitive plate on which the normal spectrum 

 is thrown is at the opposite extremity of the diameter of 

 the circular table, and the slit is also mounted on the cir- 

 cumference of the circle, in a position with respect to the 

 grating and sensitive plate which varies according to the 

 order of spectrum which is being photographed. The 

 light of the sun is thrown by a heliostat upon the slit. 

 In most positions of the slit more than one order of spec- 

 trum falls upon the plate at the same time, the red 

 region, say, overlapping the violet region of another order. 

 In order to obtain a photograph of one of these spectra, 

 say the lines in the red region, the blue light of the other 

 spectrum is cut out by a bath of absorbing liquid placed 

 in front of the slit. When all but the one spectrum is 

 eliminated, the actinic action at one end of the plate will 

 be sometimes as much as fifty times as great as that at 

 the other end ; and if both ends of the plate were exposed 

 for the same period, the one end of the spectrum would be 

 greatly over-exposed when the liaes at the other end were 

 just beginning to register themselves on the plate. To 

 avoid this difticulty Mr. Higgs makes use of shutters 

 within the camera, moved by clockwork, which can be 

 adjusted so that different portions of the plate are exposed 

 for difierent periods. Possibly on another occasion I shall 

 be able to give further details as to some of the ingenious 

 appliances which Mr. Higgs has devised to facilitate his 

 work. 



ILcttcvs. 



fThe F.ditor does not hold himself responsible for the opinions or 

 statements of correspondents.] 



Ti> the Editor of Knowledue. 



Sir, — The following note on " Recurring Decimals " of 

 the form aticJca J>^c^...l.-^ where <i + H^ = h + h^= &c. 

 = '■ - li if '• = the radix ; consisting of two equal periods 

 of n figures, /' & '.' suiiplGmcntiu-y to each other — may 

 interest your readers. 



1. Let s=-ri} 



r" .S =/'•(,)/' 

 :.(r" + l) s^r+1 



r"+l 



112 -fl 143 1 



tlius-l4'2 857 = 



lOHl ~ 1001" 7 



All fractions whose denominators are divisors of r" -f- 1 

 belong to this class ; 



thus 



1 



= v,=-09 



11 



1 



'lOl 



•GO 99 



2. All fractions of the form 



1 



lo' + r 

 1^ 



102 + 1' 



The divisors of 10'' + 1 = 1001, are 7, 11. 13, so that i, 

 Jj, &c., belong to this class, &c. 



-1 



(1) belong to the above ; 



(2) the recurring figures can be immediately ob- 



tained by multiplying from the end by n ; 



(3) or by dividing from the beginning by k. 



These methods were first pointed out bv me in Satuie 

 for 1878, page 291. 



Thus fV=-052(33l578 9473C8421 

 and can be at once written down 



(1) by multiplying by 2 from the end, the last 



figure being unity ; 



(2) by dividing by 2 from the l>eginning on a 



similar plan ; 



(3) or by writing down one period by either of the 



above ways, and adding the supplementary 

 period. 



Knowing ^^-^^ we can at once write down the period for 

 ;'7Ei l^y subtracting the former from unity or 9 in our 

 scale. 



Thus j^'tj = -052631578 I 947368421 

 la = -947368421 | 052631578 



We observe here that the supplementary period simply 

 comes first, and, its last figure being unity^ we are able to 

 perform the Hibernian feat of Iniiinnimi in the miildle 

 and working towards the left by multiplying by 2, or 

 towards the right by dividing by 2. R". CH-UiiREs. 



To the Editor <;/' Knowledge. 



Sir, — Your article in Knowledge for May is just to 

 hand, and in regard to your quotation of my remark 

 (p. 130) that " many of the rays join two volcanic centres,"' 

 I think you will see that this is correct. Neisou, on p. 77, 

 says " in some cases the rays end sharply at a crater, or 

 ring plain." On p. 314 he says : " On the'west many light 

 streaks unite the two systems of Copernicus and Kepler, 

 . . . north-east a great bundle of long, thin, very intense 

 streaks unite the two systems of Aristarchus andlvepler," 

 and this feature is referred to in other places. I should 

 hardly have risked such an important " statement " on 

 my own responsibility. 



With regard to rays being due to snow depoxiied from air 

 " cooled hi/ radiation," after issuing from long radial 

 fissures, might I point out that air — warm or cold — could 

 hardly issue into such a void so gradually as is implied ; 

 such an issue would, almost necessarily, be of an explosive 

 nature. 



To this I attribute the formation of the iiimbi or bright 

 (often rayed) patches around some craters, see p. 24 of my 

 theory — i.e. to the discharge at rare intervals of some air 

 or gases charged with aqueous vapour. 



The fact that craterlets, crater cones, and rows of con- 

 fluent craters are so generally seen on the line of clefts, 

 is a fairly good proof that, if due to exhalation of aqueous 

 vapour, they must pass down to the warmer and moister 

 suhstrntd. 



