June i6, 1898] 



NATURE 



151 



LETTERS TO THE EDITOR 

 [The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of NATURE. 

 No notice is taken of anonymous communications.^ 



The Origin of the Aurora Spectrum. 



Prok. Ramsay gives the wave-length of the principal line 

 in his new gas as 5566. It will no doubt also occur to others 

 that this is very near the wave-length of the aurora line, which 

 \"ogel has measured as 5569. It should be mentioned in con- 

 nection with this line that Profs. Liveing and Dewar have 

 observed one very near it at 557 in sparks taken in liquid 

 xygen. The second green line given by Prof. Ramsay as 

 ^557, seems also to have been seen by these observers {Phil. 

 Mag., xxxviii. p. 237, 1894). ARTHUR SCHUSTER. 



Manchester, June 10. 



The Action of Electric Discharges on Photographic 

 Plates. 



Referring to the paper on this subject, read on May 16, 

 by Mr. J. A. McClelland, at the Cambridge Philosophical 

 Society, and reported in your issue of June 9 (p. 142), perhaps I 

 may be allowed to mention that very similar experiments, with 

 the deduction that the effect is chiefly due to light, and not to 

 electrolytic or other action, were described by myself in a paper 

 to Section A of the British Association, at its Edinburgh meeting 

 in 1S92, and will be found fully reported in the ^/^f/r/Va/ i?ifz//^z£/ 

 for August 26 of that year. 



I do not know whether others have observed the fact that 

 when strong sparks from an induction coil or influence machine 

 are allowed to traverse the sensitive surface of an ordinary 

 photographic dry plate, that a dark line, delineating the path of 

 the spark, is immediately produced, and can clearly be seen 

 without any necessity for photographic development. Further, 

 that such lines, though faint to commence with, darken appreci- 

 ably after a few minutes lapse of time, and still more so in the 

 course of a few hours. This appears to indicate that whatever 

 the precise action of the spark on the film, this action continues 

 after it has once been started. Further, it is a curious fact that 

 these lines, if examined with a magnifying glass, are always 

 found to consist of two dark lines with a light space between 

 them. This is specially noticeable immediately after the spark 

 has passed, the space apparently filling up with lapse of time. 



A. A. C. SWINTON. 



66 Victoria Street, London, S.W., June 10. 



A High Rainbow. 



On Sunday afternoon. May 29, while sitting in my yard, my 

 twelve-yearold son called my attention to a rainbow which he 

 had discovered while lying on his back looking up at the sky. 

 The local time here was 5.40 p.m., and the sun, therefore, about 

 an. hour and a half high. The bow was in the west, and about 

 70 degrees from the horizon, with its convex side to the sun. 

 The colours were fairly well brought out, the red being on the 

 convex side of the arc, and the violet on the concave side. The 

 figure on p. 132 of Tait's "Light" shows a short arc near the 

 zenith, which is a fair representation of what was seen here. I 

 have not read an account of what was seen by Helvetius further 

 than is contained in Prof. Tait's book, and do not know 

 whether the arc seen by him near the zenith showed the rainbow 

 colours. In this case I do not see any of the other halos seen 

 hy Helvetius. There were but few very thin clouds, and no 

 rain at all. Sidney T. Moreland. 



Lexington, Virginia, U.S.A., June 2. 



NAUTICAL ASTRONOMY. 



T F the compass is the navigator's sheet-anchor, the 

 -»■ sextant is certainly his best bower ; and just as the 

 former was known, if not generally used in Europe, 

 about a century before Flavio Gioia got the credit of dis- 

 covering it, so the latter was invented by the trans- 

 cendent genius of Sir Isaac Newton, more than half a 

 century before it was re-invented by Hadlcy in 1731. 



NO. 1494, VOL. 58] 



Newton does not seem to have suggested its adaptability 

 for navigational purposes, or if so, it was not sufficiently 

 known or taken up, and I am not aware of any reason 

 to suspect that Hadley knew of Newton's discovery. 



The principal use the navigator puts the sextant to is 

 that of measuring the altitudes of heavenly bodies— that 

 is, the angle at his eye subtended between the object 

 and the visible horizon. Now the rational horizon may 

 be defined as the plane perpendicular to the plumb-line 

 through the earth's centre, or the circle traced by the 

 meeting of this plane with the celestial concave. The 

 sensible horizon is generally defined as a plane parallel 

 to the former through the eye of the observer ; but this 

 can only coincide with the visible horizon if the eye of 

 the observer is at the surface of the earth — as if he were 

 immersed in the sea, till a horizontal line from his eye 

 would be a tangent to the sphere at that point. But the 

 eye of the observer is always above the surface of the 

 sea ; and the more it is raised, the more the visible 

 horizon is depressed, and a correction called " dip " has 

 to be applied to an altitude measured to it, to reduce it 

 to what it would have been had the eye been at the sea- 

 level. Again, before this apparent altitude can be used 

 for position-finding, it has to be still further corrected for 



FiC.I 



refraction, due to the bending of the rays of light, in 

 passing through the earth's atmosphere, and in the case 

 of sun, moon, or planet for parallax, to reduce it to the 

 angle at the centre of the earth and to the rational horizon. 

 Both these corrections are zero when the body is in the 

 zenith, and a maximum at the horizon. Parallax is the 

 angle at the observed body, subtended by the semi- 

 diameter of the earth under the feet of the observer, 

 which will be reduced to a point when the body is in the 

 zenith. If the body has an appreciable semi-diameter, it 

 has to be applied to the altitude of the limb to get that 

 of the centre. 



In the diagram (Fig. i), let H E z P R Q N p' represent a 

 meridian of the celestial concave, and the inner circle 

 the corresponding meridian of the earth ; let z be the 

 zenith, N the nadir, P and P' the poles of the heavens, 

 being the points in the celestial concave, which would be 

 perforated by the earth's axis if indefinitely produced : 

 then H R will represent the rational horizon, the plane of 

 which, passing through c, is normal to the plumb-line 

 z o N, J O iJ will represent the sensible horizon (o being 

 the position of the observer), E Q, the plane of which is 

 nomial to P P', will be the equinoctial, whose plane co- 

 incides with that of the terrestrial equator. On a meridian 



