6i6 



NA JURE 



[April 27, 1899 



OUR ASTRONOMICAL COLUMN. 

 Astronomical Occurrences in May : — 

 May 2-6. Meteoric shower before sunrise from Aquarius. 



4. Tullle's cornel in perihelion. 



5. iih. 6m. Minimum of Algol (/3 Persei). 



13. Sh. 47m. to loh. 12m. Transit of Jupiler's Sat. III. 

 16. Swift's comet closely S.W. of o Andromediv (mag. 



3-8)- 

 20. I2h. 5m. to I3h. 36m. Transit of Jupiter's Sat. 

 III. 

 loh. 15m. to iih. 25m. Oecultationof B.A.C. 5254 



(mag. 5-4) by the moon. 

 Ilh. 39m. to I2h.39m. Occultation of 7 Sagiltarii 



(mag. 54) by the moon. 

 I2h. 3m. to I3h. 15m. Occultation of 9 .Sagittarii 

 (mag. 57) by the moon. 

 2h. Uranus in opposition to the sun. 



Tuttle's Comet (1899 /')• — The following positions are 

 continued from the ephemeris by Ilerr J. Kahts in Aslr. Nadi. 

 (Bd. 149, No. 3555) :— 



Ephemeris for 12A. Berlin Mean Time. 



1899. 



•89 



I 90 



Tempei-'s Comet (1873 II.). — The following search ephe- 

 meris is given by M. L. Schulhof in Asl. Nae/i. (Bd. 149, No. 



3554) :— 



Ephemeris for \lh. Parts Mean Time. 



o'46o 



Double-star Catalocue.— .^.r/. Na,h. (Bd. 149, Nos. 

 3557 and 3558) contain a catalogue of 132 new double-.stars 

 which have been discovered by Prof. G. \V. Hough, with the 

 l84-inch refractor of the Dearborn Observatory, Illinois, U.S.A. 

 This is the fourth catalogue of new double-stars issued from the 

 same observatory, the total number now reaching 622. The 

 measures have been made during the period 1S94 to 1897, each 

 pair having been measured on two or more nights. Magnifying 

 powers of 390 and 925 have been most freijuently employed. 

 It is noted that one of the stars. No. 580 in the catalogue, is 

 probably a binary, successive measures having indicated motion 

 between the components. This star is Lalande 37881, and its 

 position for 18S0 is R.A. I9h. 47m. 19s.; Decl. -f 22° 9'. 

 The change of position angle, on which the probability of its 

 being a binary is based, is .shown in the following table : — 



Date. 

 189576 

 1897-62 



Law of Tf-MI-eraturE in Gaseous Boihes.— The Astro- 

 nomical yournal (tio. 459) contains some further criticisms on 

 Dr. See's article on "The Sun's Heat"(W. J., No. 455) ^'V 

 Mr. C. M. Woodward, of Washington University. The points 

 he brings forward are as follows : — 



( 1 ) Dr. See assumed his hypothetical gaseous glolx; to have a 

 definite boundary, but there is no good reason to suppose that 

 a pure gas, unrestrained save by the mutual attraction of its 

 particles, has a definite limiting surface. 



(2) The assumption that the pressure at K,, is directly 



NO. 1539. VOL. 59] 



measured by the weight of an element of mass is wrong ; it is 

 the variation in the pressure which is measured by the weight. 



(3) Instead of the intensity of pressure varying inversely as 

 the fourth power of the radius, as given by Dr. See, it really 



varies inversely as the square, or P = — . 



K. 



the value of P, by which it is determined, is wrong. " Dr. 

 See appears to forget that when the volume of a given gas is 

 fi.\ed by other considerations, the pressure is independent of the 

 force of gravity. He leaves the question of temperature still 

 unsolved. Instead of finding the temperature from the pressure, 

 the temperature is to be determined by the principle that the 

 change of teiuperature during contraction must be such as to 

 render the force of mutual attraction sufficient to do the work of 

 compression. I'or a solution of that problem, I beg leave to 

 refer to my paper read before the Saint Louis Academy of 

 Science, March 20, 1S99." 



Return oi' Hol.mes' Comet (1892 III.). — Mr. H. J. 

 Zwiers gives in Ast. Nach. (Bd. 149, No. 3553) a detailed 

 ephemeris for the e.\pected return of this comet, which he 

 calculates will pass perihelion on April 27. This not being 

 quite certain, he also gives the positions corresponding to peri- 

 helion passages on April 19 and May 5. We append the 

 positions for T = April 27 '665 1 : — 



Ephemeris for I2h. G.M.T. 



THE THEORY OF THE RAINBOW. 



A TOST text-books still explain the rainbow phenomena with 

 •'■■'■ the aid of Descartes' "effective" rays. This explan- 

 ation concerns only the principal and the secondary bows ; it 

 does not take notice of the supplementary bows, nor of the 

 fact that the colours, their intensity, breadth, and sequence 

 vary greatly with the size of the r.iindrops. That the rain- 

 bows are interference phenomena, was recognised by Young. 

 Their correct theory is contained in Airy's " Intensity of Light 

 in the Neighbourhood of a Caustic" (Trans. Cam. Phil. Soe., 

 vi. p. 379 ; viii. p. 595, 1838 and 1848). The intensity de- 

 pends upon an infinite integral, which Airy first attempted to 

 coinpute by iiuadratures. Stokes found a more convenient 

 form for his integral. The theory was first verified by W. 

 Hallows Miller in 1S71 : later by Bottel, Mascarl, ai\d others. 

 In 1896, Prof. 1. M. I'ernler, then at Innsbruck, now director 

 of the Central .Station for Meteorology and Karth Magnetism 

 at Vienna, presented a long paper on the colours of the rain- 

 bow and the while rainbow to the Vienna Academy ( lyien. 

 Al-ad. Jier., 106, ii. ,1, p. 137, 1897). The paper involved very 

 laborious calculations and experiments, and dealt fully with the 

 infiuence of the si/e of the raindrops; it was referred 'to in 

 Nature, January 27, 1898. In the Jubilee number of the 

 Zeilsehrift fiir Ocstcrreichisehe Gymnasien, December 1898, 

 Pernter has now made an attempt to render Airy's theory 

 intelligible to the pupils of secondary schools. 



The importance of the angle of minimum deviation is ex- 

 plained with the help of the diagram (Fig. I). Not to com- 

 plicate the figure, the rays are all supposed to correspond ap- 

 proximately 10 Iraunhofer's line C in the red-orange. With 

 increasing angle of incidence, the deflection of the refracted 

 ray from its original direction becomes greater and greater, 

 until a maximum is reached (the dotted ray) ; on further in- 

 creasing the angle of incidence, the deviation decreases again. 



