438 



NATURE 



{Oct. I, 1874 



one another. The fainter series are'attributed to interference. In 

 bright rainbows there are three, if not four, series of colours, at 

 least in the upper part of the arch, where the colours are always 

 the most distinct, probably owing to the rain-drops being smaller 

 high up, and therefore more perfectly globular. It may not be 

 generally known that a rainbow may be seen much more per- 

 fectly in a single drop of dew, by placing the eye close to it, 

 than in rain, and then no less than ten or twelve series of colours 

 may be seen ; and in the irregular dew-drops (as also in hoar- 

 frost) a great and very beautiful variety of bows and spectra can 

 ^,e seen. T. W. B.\CKHOUSE 



Sunderland, Sept 23 



I SHOULD like to say a lew words regarding Mr. Swettenham's 

 letter (Nature, vol. x. p. 39S). The mathematical theory of 

 the rainbow has been worked out pretty completely. We must 

 not look for it, however, in text-books, which generally give a 

 very unsatisfactory account of the rainbow, but m the original 

 memoirs, which sometimes are very difficult to find. 



The appearance of coloured bands inside the primary rainbow 

 is not at all of very rare occurrence ; since my attention has 

 been drawn to them by a casual observation, I have seen them 

 repeatedly. Only a few weeks ago I saw distinctly three con- 

 centric bows, with all the colours inside the primary bow. These 

 bows have been called supernumerary rainbows. The complete 

 mathematical theory has been given by the Astronomer Royal in 

 the Philosophical Magazine, and the theory has been verified by 

 Mr. Miller. The cause of these coloured rings is the interfer- 

 ence of two rays of light entering the rain-drop at diflerent angles 

 of incidence, but having the same deviation, and therefore leaving 

 the rain-drop parallel to each other. It is clear that two such 

 rays must exist for all deviations from the maximum to^the devia- 

 tion of ray of light having an angle of incidence of 90°. 



In text-books no mention is ever made of these supernume- 

 rary rainbows, and this is the more to be regretted as the inter- 

 ference mentioned above is, I think, one of the principal causes 

 of its formation. 



Were the explanation given in text-books complete, we should 

 not have in the rainbow such pure colours as we actually see, 

 but the yellow would contain a great deal of red, and the green 

 would be contaminated by a great (juantity of red and yellow. 

 As it is, however, the red, which would have the same deviation 

 as the green and yellow rays, is destroyed, or nearly so, by inter- 

 ference, which, therefore, is the cause that the colours of the 

 rainbow are nearly pure. What is called the violet of the rain- 

 bow is generally the violet mixed with the red of the next super- 

 numerary rainbow. This is not the only instance that text-books 

 contain incomplete accounts of phenomena which have been 

 satisfactorily explained. Arthur Schuster 



Sunnyside, Upper Avenue Road 



Mist Bows 

 On Sept. 14 I was driving Irom the Lizard just after sunrise 

 with Mr. Lugg of Manaccan. A thick mist covered the fields 

 and moorland. The tops of the farm buildings and corn stacks 

 and the church towers were visible above the sea of mist which, 

 matted on the ground, gave the entire country the appearance of 

 being covered with snow. About 6.30 A.M. the sun was bright 

 on our right hand, and on the left we saw a halo of prismatic 

 colours forming a distinct circle of rainbow at a little distance 

 from and encircling the shadows of our heads, and only broken 

 where the shadows of our bodies interposed. This appearance 

 lasted for ten minutes, and our shadows with their attendant 

 bow showed brightly against the mist background as we passed 

 hedges and fields, and kept pace with us like " the mist raised 

 from the plashy earth" by the hare in Wordsworth's poem, 

 *' Thiit, glittering in the sun, 

 Runs witli her all the way wherever she doth run." 

 We afterwards opened a valley terminating in an extensive 

 moor, when the mist appeared as a sea of prismatic colour extend- 

 ing to the horizon. About 7 A.M. we saw a perfect bow free 

 froin any prismatic colour, both ends of which terminated in the 

 field immediately to our left. 



My companion, who is constantly driving .about this district in 

 early morning, says he never before saw similar phenomena. 

 Lizard Signal Station, Sept. 16 Howard Fox 



Carnivorous Plants— how to be obtained 

 1 1 is not unlikely that there may be a great demand for plants 

 of the genus Drosera, and as I am in a neighbourhood where 



the supply of the D. rotundifolia and D. intermedia is inex- 

 haustible, I shall be glad to send, through the post, plants of the 

 same to any who are desirous of investigating their carnivorous 

 habits ; but to meet the necessary expenses of collecting and 

 postage, six penny stamps must be enclosed in the application 

 for each dozen plants. The applications of dealers in plants 

 will not be attended to. 



The D. intermedia is far more abundant than the D. rotundi- 

 folia, and will answer the purpose of investigators quite as well. 

 A few words about the method of growth of these may not be 

 out of place. Pure peat well soaked with water suits either 

 kind, but while the D. intermedia flourishes with its roots be- 

 neath the surface of the water, D. rotundifolia grows best when 

 it is from 3 in. to 4 in. above the surface ; now and then it 

 happens that it is found with its roots in the water, and then the 

 hairs on the stalks of the leaves, which constitute one of the 

 distinguishing features between these species, are much dimi- 

 nished, both in number and length. 



The Liverpool naturalists will find alarge supply of the D. ro- 

 tundifolia on Oxton Common, and there they are most abundant 

 in the corner, nearest to Noctorum Farm. Thurstaston Hill is 

 another locality in the same neighbourhood where this plant 

 grows. 



The Pingiiiada lusitanica is not uncommon in the bogs of the 

 New Forest, but 1 cannot promise specimens of this plant with 

 the same certainty as I can those of the Droser.x-. Applications 

 for plants had better have the word Drosera written on the enve- 

 lope, to prevent the delay which would arise from such letters 

 being forwarded to me when away from the New Forest. 



Bisterne Close, Burley, Hants G. XL Hopkins 



[Both species are moderately abundant, though small, in a 

 peat-bog near Bumham Beeches, Bucks, about four miles from 

 Slough.— Ed.] 



Automatism of Animals 



ProI''. Hu.xley's most interesting address published in 

 Nature, vol. x. p. 362, seems to me to involve some difficulty, 

 which I take the liberty to state, though well aware that 1 am 

 stepping on slippery ground. I allude to this p.assage : — " Sup- 

 pose I had a frog jilaced in my hand, and that I could make it, 

 by turning my hand, perform this balmcing movement. If the 

 frog were a philosopher he might reason thus : ' I feel myself 

 uncomfortable and slipping, and feeling myself uncomfortable I 

 put my legs out to save myself. Knowing that I sh.all tumble if 

 I do not put them further, I put them further still, and my voli- 

 tion brings about all these beautiful adjustments which result in 

 my sitting safely.' But if the frog so reasoned he would be 

 entirely mistaken, for the frog does the thing just as well when 

 he has no reason, no sensation, no possibility of thought of any 

 kind." 



Now, does it unavoidably follow from the latter fact that this 

 philosophising frog would be entirely mistaken ? What I should 

 venture to object is simply this : — Experiment shows, indeed, that 

 very delicate combinations of muscular actions (as in swimming) 

 are brought about by impressions upon the sensory nerves, even 

 when, alter ablation of the btain, there can be no longer any 

 consciousness. But have not those combinations originally 

 arisen during undisturbed consciousness, and therefore, perhaps, 

 under the influence of consciousness, inscrutable as the relation 

 of consciousness to corporeal phenomena is acknowledged to be? 

 And even if the experiments alluded to should succeed with 

 animal individuals which, before vivisection, never h.ad executed 

 the movements in question (and I was once assured by a distin- 

 guished physiologist that similar experiments do really succeed 

 with rabbits depri\xd of part of brain soon after birth), yet those 

 adjustments may be rather considered with regard to the great 

 principle'_of inheritance, as it has been apphed to instincts by Mr. 

 Darwin and Mr. Spencer, and alluded to in I'rof. Tyndall's 

 address. Though now performed by animals without possibility 

 of sensation and thought, those movements were adjusted to 

 each other, and to imiiressions on sensory nerves in these ani- 

 mals' ancestors while in possession of consciousness. 



Surely such questions will ever remain doubtful ; yet I think 

 it not unbecoming to state a view of them which seems to me 

 to be in accordance _ with the present direction of biological 

 thci'ries. I. 1>. Wetterhan 



FrankforL-on-the-Maine, Sept. 20 



