September 6, 191 7] 



NATURE 



they profess themselves socialists is ease without 

 effort." "The advertisement of cheap and pain- 

 less substitutes for war has been a recurring 

 feature in the cycles of civilisation." We suspect 

 that the author has a sense of humour hidden away 

 somewhere and tha.t perhaps he does not really 

 mean all he says. 



IDENTIFICATION OF PLANTS. 



me this Flower: A Simple Way of Finding 

 at the Names of Common Plants without any 



Previous Knowledge of Botany. By Prof. 



Gaston Bonnier. Pp. xii + 33 1 + plates 64. 



(London and Toronto: J. M. Dent and Sons, 



Ltd. ; New York : E. P. Dutton and Co., 191 7.) 



Price 65. net. 



THE desire to know the names of wild flowers is 

 very widespread and by no means confined to 

 those who take any particular interest in botanical 

 science. For such as these there has been no 

 easy book of reference. The simple books have 

 all been written on botanical principles, and the 

 science of plant classification underlies almost 

 every so-called popular treatise. 



Prof. Gaston Bonnier has fully realised this, and 

 in producing his book, "Name this Flower," has 

 achieved a really useful purpose. At first the 

 botanist may be tempted to scoff and consider it 

 a wasted effort, for the construction of the admir- 

 able keys must have been a most laborious work. 

 But a little study reveals its great value, and a 

 test with such difficult plants as sea holly or teazel 

 shows how thoroughly the work has been done. 



In writing the book Prof. Bonnier was largely 

 influenced by the philosopher Ernest Bersot's 

 "Letter on Botany," published among his "Reflec- 

 tions of a Moralist." "Botany," he says, "is 

 one of the most deceitful of sciences. As flowers 

 are so charming one imagines that it also must 

 be charming ; but how soon one is disillusioned ! 

 And why? Ah, why? Because the savants have 

 thought about themselves and not about us. They 

 have wished for a science complete in itself; and 

 they have put each thing in its place without 

 troubling to ascertain whether it would be easy 

 for other people to find it there. How many 

 times have I tried to become a botanist, and 

 each time I have been vanquished." 



Prof. Bonnier, by his exhaustive keys, well 

 illustrated by line drawings, enables anyone to 



: find out the names of plants without knowing 

 anything of botany or of the principles of classifi- 

 cation. The value of the book is enhanced b"V 

 sixty-four plates of coloured illustrations, which 

 represent the plants sufficiently adequately. A 

 P^ood deal of botanical and general information 

 IS also packed into the book, and it is very well 

 indexed. Anyone using Prof. Bonnier's book 

 carefully could scarcely fail to find that in so doing 

 he had not only learnt the names of plants, but 



I was also being imp>elled on the high road to 

 become a botanist. 



English students should be grateful to Prof. 

 Boulger for this translation of Prof. Bonnier's 

 excellent book. 



NO. 2497, VOL. 100] 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for 

 opinions expressed 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.] 



Unusual Rainbows. 



The very interesting diagram sent to Nature for 

 August 30 (p. 525) by Mr. Allan Low seems to exhibit 

 a complete system of direct and reflected rainbows. 

 The falling- raindrops would be flooded with the direct 

 light of the sun, forming the usual pair of bows. They 

 would also be flooded with the light reflected from 

 the surface of the sea ; if the sea is not ruffled this 

 latter light would be directed from a fairly concentratetl 

 image of the sun, below the horizon. Thus it should 

 show another pair of bows, the common axis of which 

 is the prolongation of the line from the observer to 

 this image of the sun ; as that line points above the 

 horizon, these bows should be more than a semicircle 

 in extent. When the surface of the sea is ruffled, the 

 blurred image of the sun will be so large that the 

 colours in the bows will overlap, and only a broad white 

 bow will appear, which would not be noticed. Fog 

 bows are white for a different reason. The two sys- 

 tems of bows meet at points which must be equidistant 

 from the sun and its image; for bows of the same 

 radius these points must be on the horizon. The alti- 

 tude of ' the anti-sun, the radius of the bow 

 drawn to the horizon, and the horizon form 

 a right-angled spherical triangle ; thus the sine 

 of half the angle between two bbws where 

 they intersect on the horizon is equal to the sine of the 

 sun's altitude divided by the sine of the radius of the 

 bow. With Mr. Low's estimated figures this would 

 bring out the radius rather too small. J. L. 



Cambridge, August 31. 



The arcs of the third and fourth bows, so well 

 described by Mr. Low in Nature of August 30, are, I 

 think, undoubtedly due to the sun reflected from the 

 ocean behind the ship. 



Around a centre O describe two circles with radii 

 of 42 mm. and 52 mm. respectively. Then 7 mm. 

 above O draw a horizontal line. This will represent 

 the horizon, and the portions of the circles above this 

 will be the primary and secondary bows due to the 

 direct light of the sun. On a line from O, perpen- 

 dicular to the horizontal line, take another point P, 

 distant 14 mm. above O, and describe circles about P 

 with radii as before. The portions of these circles 

 above the horizontal line will be the primary and 

 secondary bows due to the reflected sun. It will be 

 found that the figure thus obtained is very similar to 

 that given by Mr. Low, except that he saw only small 

 portions of the third and fourth bows. But I believe 

 the sun's altitude must have been greater than 7°, for 

 with that height only about one-sixth of the vertical 

 radius of the primary bow would be below the horizon, 

 and in his diagram about one-third is cut off. I have 

 taken the radii of the primary and secondary bows as 

 about 42^ and 52°, in round numbers. 

 Invermay, Hyde Park, Leeds, .August 31. 



C. T. Whitmell. 



There seems to be a simple explanation of " .An 

 Unusual Rainbow " described by Mr. Low in Nature 

 of .August 30. If the sea was sufficiently calm there 

 would be, from the point of view of the raindrops 

 causing thj rainbow, a real sun 7° above the horizon 



