34 



NATURL 



[March 9, 19 16 



suggestion and determine the probable error of each 

 experimental result? Each result could then be ex- 

 pressed by a circle the radius of which is equal to 

 half the probable error, and which would increase with 

 ithe size of the diagram. If another experiment be 

 made under similar conditions it is about an equal 

 chance that it falls within or without the circle, which 

 therefore affords a measure of the precision of the 

 observations. Since there is little evidence against 

 any curve which cuts the circle, the variations in size 

 might profoundly modify the opinion of the drafts- 

 man as to the direction of his curve. 



Sydney Lupton. 



Ground Rainbows. 



I HAVE seen with pleasure Mr. Heath's clear and 

 instructive letter and diagrams on this subject in 

 N.-VTURE of March 2. Some fourteen years ago I cal- 

 culated the altitudes of the sun required to produce 

 the elliptic and other arcs, and obtained results in 

 agreement with Mr. Heath's, except that I took 41° 

 instead of 42° for the semi-angle of the cone. 



For Petersfield, at 11 a.m. on October 14, 1915, 

 the sun's altitude, 23°, appears to be somewhat under- 

 estimated, and I make it just above 30°, but this, of 

 course, leaves the bow still hyperbolic. 



I was led to consideration of the curves for the 

 ground rainbow when seeking for a reason why the 

 sky rainbow is seen always circular, though, when the 

 sun is not on the horizon, the bow might perhaps 

 have been expected to appear elliptical, the circle being 

 projected into an ellipse on a plane perpendicular to 

 a sight-line, assumed horizontal. 



I came to the conclusion that, there being no definite 

 plane of reference in the sky, and the rays being 

 parallel, there is, as it were, no element of definite 

 distance involved, so that the sky bow always appears 

 circular. But for the ground bow we have a definite 

 horizontal plane of reference, so that this bow becomes 

 a conic section, varying with the sun's altitude. 



I had some interesting correspondence at the time 

 with the late Sir G. G. Stokes, and I may perhaps 

 quote from one of his letters, dated August 22, 1902, 

 only six months before his death. Replying to my 

 question as to whether a dew bow is seen as a circle 

 or an ellipse, he wrote : — 



" It is a question of the combination of sensation 

 and expectation. In a dew bow we are impressed with 

 the idea that the luminosity we see is spread over a 

 horizontal plane ; and we tacitly ask ourselves the 

 question : What must be the actual form of the locus 

 of the drops on the grass in order that the luminosity 

 mav appear as it does? The answer, of course, is, an 

 ellipse, or it might be an hyperbola. If the question 

 be : As what do we see the bow? the answer depends 

 on a combination of sensation with interpretation of 

 sensation. If we merely saw the luminosity, and 

 knew absolutely nothing about its history, we should 

 never think of anything but circularity about it." 



I have often looked for a ground bow, but have 

 never been fortunate enough to see one. 



Observing a fine lunar rainbow on January 21, I 

 found the light to be polarised in planes passing 

 through the point looked at and the radius at the 

 point, just as is the case with the solar rainbow. I 

 hope that Mr. Heath will test the next, eround bow 

 with a NIcol prism. C. T. Whitmeu.. 



Invermay, Hyde Park, Leeds, March 3. 



In the Proceedings of the Roval Society of Edin- 

 burgh, vol. vli. (1869-70) Clerk Maxwell has a short 

 note on a bow .seen on the surface of ice. This was 

 observed on January 26, 1870, on the frozen surface of 



NO. 2419, VOL. 97] 



the ditch which surrounds St. John's College, Cam- 

 bridge. Maxwell remarks, " How a drop of water can 

 lie upon ice without wetting it and losing its shape 

 altogether 1 cannot profess to explain." in 1898, in 

 vol. xxii. of the same Proceedings (1898) there is a 

 note on dew bows by Dr. R. A. Lundie and myself. 

 These were produced at night on the ground, the 

 source of light being the gas lamp or electric light 

 of the street. .\ short account will be found in N.\ture 

 of Januarv \2, 1899 (vol. lix., p. 263). 



C. G. Knott. 

 Royal Socieiy, Edinburgli, March 4. 



Science and the State. 



Referring to Prof. Cohen's letter in Nature of 

 March 2, it may not be untimely to cite another para- 

 graph written in 1831 re neglect of science in this 

 country. Sir David Brewster, in his "Life of New- 

 ton," published In that year, says : — 



" But what avails the enthusiasm and efforts of 

 individual minds in the Intellectual rivalry of nations? 

 When the proud science of England pines In obscurity, 

 blighted by the absence of the royal favour, and of the 

 nation's sympathy — when its chivalry fall unwept and 

 unhonoured — how can it sustain the conflict against 

 the honoured and marshalled genius of foreign lands?" 



The position to-day is fortunately not quite so bad 

 as here indicated by Brewster, but is it not still the 

 case that, in the words of Sir Archibald Gclkie, science 

 rests under an incubus of apathy and indifference? 

 Expansion of science and national evolution are two 

 matters that In the opinion of the writer are intimately 

 bound up one with the other. Neglect of the former 

 really means inhibition of political progress. 



David Balsillie. 



Greyfrlars Garden, St. Andrews, March 4. 



THE NATIONAL IMPORTANCE OF THE 

 DYE INDUSTRY. 



AT the annual meeting of the Bradford Dyers'" 

 Association held on February 28 the chair- 

 man of the directors, Mr. Milton S. Sharp, made 

 a highly interesting- statement on the national 

 position with regard to the supply of dyes. He 

 described with great force and clearness the close 

 connection between the manufacture of dyes and 

 high explosives, and pointed out how Germany 

 by reason of her huge, highly organised, and ably 

 administered colour works, producing all the raw 

 materials for the making of high explosives, was 

 able immediately to divert much of their plant 

 to war purposes. He paid a high tribute to Lord 

 Moulton and the High Explosives Department for 

 their services, the value of which, he said, the 

 country will probably never know-, in improvising 

 the manufacture of high explosives. He urged that 

 whatever it involves, we must establish the aniline 

 dye industry in this country, so that in case of 

 war we may have the ability to produce quickly 

 any amount of high explosives the Army or Navy 

 may need. The extensions of plant that have been 

 made for the temporary purpose of manufacturing- 

 high explosives will, he says, make a long and 

 essential step towards the colour industry, and to 

 break them up after the war would be little short 

 of criminal folly. Mr. Sharp quoted some effective 

 examples of German activity in relation to the 

 chemical service of the war. He alluded to one 



