422 Miscellaneous Intelligence. [Sept. 



" This phenomenon is entirely one of perspective. Let us suppose beams in- 

 clined to one another like the meridians of a globe to diverge from the sun, as 

 these meridians diverge from the north pole of the globe, and let us suppose that 

 planes pass through all these meridians, and through the line joining the observer 

 and the sun, or their common intersection. An eye, therefore, placed in that line, 

 or in the common intersection of all the fifteen planes, will see the fifteen beams 

 converging to a point opposite to the sun, just as an eye in the axis of a globe 

 would see all the fifteen meridians of the globe converge to its south pole." 



Having observed in Sir David Brewster's treatise on optics (published last year 

 in Lardner's Cyclopaedia), the above repetition of his usual account of a beautiful 

 appearance which frequently adorns our Indian evenings, and having always 

 thought his explanation of it deficient in the clearness and truth which generally 

 characterize his writings, I shall venture to suggest another mode of illustration : 



The diverging beams which to us appear to radiate from the sun when obscured 

 by broken masses of cloud are in fact, as a consequence of the sun* s enormous dis- 

 tance, sensibly parallel to each other. It need hardly be remarked that in per- 

 meating the atmosphere they do not proceed in straight lines, but are more or less 

 inflected towards the earth. This inflection being disregarded, they may be consi- 

 dered as a series of elongated parallel prisms, extending perhaps 50° to the north 

 and south of the zenith, at the height of a few thousand feet. The beam which 

 crosses the zenith will have to the spectator the same appearance as the sky seen 

 from the centre of a long narrow street of lofty houses : at the zenith it will have 

 its greatest breadth, and it will taper and appear to descend towards the sun and 

 opposite point of the heavens, and the same will happen with all the other beams 

 lying north and south of it ; whence they will appear to converge. 



I may remark, that the phenomenon is incomparably more vivid in this climate 

 than in Britain, that it is frequently and most clearly seen when there is not a 

 visible cloud except the few masses in the west which produce the necessary ob- 

 struction of the solar rays, and that the beams can be traced across the whole con- 

 cave in alternate shades of the purest light and dark blue. D. B. 



9. Errors in Dr. Arnott\ Physics, Second Volume. 



In the first volume of the Gleanings in Science I remarked on some mistakes 

 which disfigure Dr. Arnott's excellent Elements of Physics ; and the first part of 

 the second volume of the latter work having recently reached me, I would beg 

 to direct the author's attention to the following slight blemishes, which might, if 

 uncorrected, affect its otherwise well sustained character : 



The most serious mistake is in his theory of the camera lucid a, at p. 304 : the 

 eye does not see the paper " through" the prism but beyond its edge, the plane in 

 which lie the line of sight and the edge of the prism bisecting the pupil. 



At p. 218 he states that if the insertion of the optic nerves were at corresponding 

 parts of the eyes, a black spot would always be seen in the field of vision ; — for- 

 getting that when only one eye is open no black spot appears. Why no black spot 

 in this case appears is as yet unexplained. 



At p. 194 he represents as a straight line a ray of light passing obliquely through 

 the centre of a lens. 



At p. 157 he states that " the inhabitants of India, where the thermometer 

 sometimes stands at 115° in the shade, have their blood no higher than 98°." Dr. 

 John Davy from numerous experiments concluded, that blood heat in tropical 

 countries is raised to 100° during the hot season. 



