230 



NATURE 



[January 3, 1901 



It is, indeed, so far as the present writer is aware, the 

 ■only modern work which deals with the construction of 

 'large coils from a thoroughly practical standpoint. It 

 ■describes in every detail the making of the apparatus, 

 arid contains much valuable information as to the 

 general design of coils, the methods of winding and the 

 processes of insulation, which hitherto have been the 

 carefully preserved secrets of the very few makers of 

 coils of the more powerful descriptions. Questions of 

 •cost are not omitted, while special chapters are devoted 

 to contact breakers of the mercury, hand and electrolytic 

 ■types. 



The discovery of the Rontgen rays, and the im- 

 portant application that these have found in surgical 

 and medical practice, together with the increasing 

 •employment of high tension electrical discharges in 

 wireless telegraphy, spectroscopic analysis and other 

 fields, have brought about a great demand for Rhumkorf 

 coils of large size. The need for a book such as the one 

 under review has therefore become increasingly felt of 

 late years, and the only matter for regret is that the 

 -author did not give to the public the results of his 

 ■experience at an earlier date. 



The book is clearly written, well printed and well 

 illustrated. A. A. C. S. 



The Structure and Life-History of the Harlequin Fly 

 {Chironomus). By L. C. Miall, F.R.S., and A. R. 

 Hammond, F.L.S. Pp. viii + 196 ; plate and text 

 illustrations. (Oxford : Clarendon Press, 1900.) 

 The perfect insects of Chironomus are conspicuous ob- 

 jects on our windows, or may be seen dancing in swarms 

 in the open air, and are often called " gnats," to which 

 ■they have considerable resemblance ; and, like gnats, 

 the antennie of the males are very plumose. The larvae 

 are found at the bottom of standing or slowly-running 

 water, and those of some species are known, from their 

 colour, as " blood-worms," while those of other species 

 are green. The insects are easily collected and reared, 

 and present many points of interest ; and the work before 

 us gives a very clear and fairly elaborate account of the 

 structure and habits of these insects in their various 

 stages. The life-histories of insects present an inex- 

 haustible field for the investigations of any observers who 

 care to devote their attention to this branch of ento- 

 mology ; and books like the present will give the be- 

 ginner a very good idea of the best way to work on 

 similar lines. Hitherto the Diptera, though one of the 

 largest orders of insects, have been strangely neglected 

 in England, and we have not even a good descriptive 

 book on the order, though almost every Continental 

 country has a good monograph in its own language. 

 The interest felt in mosquitoes, however, at the present 

 time will probably spread to other insects of the same 

 •order ; and thus we are likely to see the study of their life- 

 histories leading to that of the Diptera as a whole, in- 

 stead of interest in the order generally, leading to re- 

 searches into its life-histories, as has been the case with 

 some of the other orders of insects. 



The bulk of this work is too technical and too elabo- 

 rate to admit of its being discussed in detail, and it 

 contains much useful general information relating to 

 allied species, nor are the parasites of the larvae left 

 unnoticed. One remark strikes us as specially interest- 

 ing : " No insect is known to us which has more com- 

 pletely departed from the habits and structure of an air- 

 breathing animal. Yet even here we find visible proof 

 of descent from a terrestrial insect with branching air- 

 tubes." 



In an appendix we find a section on " Methods of Ana- 

 tomical and Histological Investigation," and an additional 

 ■note by Mr. T. H. Taylor on the swarming and buzz- 

 ing of Harlequin flies. The book concludes with a good 

 ■bibliography and index. W. F. K. 



NO. 1627 'VOL. 63] 



LETTERS TO THE EDLTOR. 

 \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.^ 



On the Nature of the Solar Corona, with some 

 Suggestions for Work at the next Total Eclipse. 



In an article on the corona, published in the November 

 number of the Astrophysical fournal, I suggested a method by 

 which the existence of the Fraiinhofer lines in the spectrum of 

 the corona might be detected. The method was based on the 

 supposition that the light emitted by the particles, in virtue of 

 their incandescence, so overpowers the reflected sun-light that 

 the lines are invisible. That the coronal light is strongly 

 polarised is well known, and there is scarcely any doubt but 

 that the polarised light is reflected sun-light. If, now, a Nicol 

 prism be placed before the slit of the spectroscope in such a 

 position as to transmit the polarised radiations, these will be 

 allowed to pass with almost undiminished intensity, while the 

 emitted or unpolarised light will be reduced in intensity by one- 

 half. The great change in the ratio resulting might easily be 

 sufficient to bring out the dark lines distinctly. I feel firmly 

 convinced that this experiment should be tried at the Sumatra 

 eclipse of next May, lor I have successfully accomplished it in 

 the laboratory with an artificial corona. It was found that a 

 gas flame in a strong beam of sun- light shone with a pure bluish- 

 white light, due to the reflection or rather scattering of the 

 sun-light by the minute carbon particles/' The flame thus 

 illuminated showed the Fraimhofer lines distinctly, but by 

 reducing the intensity of the sun-light a point was reached at 

 which they disappeared, and the spectrum appeared continuous. 

 The light scattered by the flame was found to be completely 

 plane polarised in certain directions, giving us just the required 

 conditions, namely, particles emitting a continuous spectrum, and 

 scattering a polarised solar spectrum. In front of the slit of the 

 spectroscope a Nicol prism was arranged in such a manner that it 

 could be drawn into and out of position by a cord. The 

 Fraiinhofer lines could be made to appear by sliding the Nicol 

 in front of the slit, and disappear by drawing it away. While 

 it does not by any means follow that the use of a Nicol on the 

 actual corona will bring out the lines, the experiment seems to 

 be well worth trying, as it would furnish further information re- 

 garding the relative intensity of the emitted and reflected light. 

 Another interesting point is that the minute particles in the 

 flame do not scatter the longer waves, the flame reflecting 

 practically no red or orange light. Thus the Fraiinhofer lines 

 can only be traced to about the D lines. By reducing the 

 intensity of the sun-light they disappear, first in the yellow, then 

 in the green, blue and violet in succession. This indicates that 

 our chances of detecting the lines in the spectrum of the corona 

 will be greatest in the photographic part of the spectrum. 

 Moreover, it appears to explain the absence of radiant heat in 

 the light sent to us from the corona, the particles being too 

 small to scatter these longer waves to any appreciable extent. 

 Abbott, of the Smithsonian party at Wadesboro', found the 

 corona cold in comparison with his bolometer, and infers from 

 this that the corona neither reflects sun-light nor emits light in 

 virtue of incandescence, expressing the opinion that the lumin- 

 osity is analogous to that of vacuum tubes transmitting electric 

 discharges. It seems to me that the polarisation of the coronal 

 light makes this theory untenable, and that the absence of heat 

 rays can be explained fully by the small size of the particles. I 

 am aware that the absence of radiant heat in the emitted light 

 has yet to be accounted for. My own notion, based on experi- 

 ments which are now in progress, is that the reflected or 

 scattered light is vastly in excess of the emitted, and that the 

 absence of the Fraiinhofer lines is more probably due to the line- 

 of-sight motion of the particles than to simple drowning out by 

 emitted light. 



My experiments on the ratio of emitted to scattered light of 

 a body brought to incandescence by powerful solar radiation are 

 not yet completed, consequently I do not yet feel prepared to 

 make any very positive statement in regard to this matter. A 



i Since writing the above I have found that the reflection of light by a 

 flame has been described by Mr. Burch and Sir George Stokes indepen 

 dently. It was noticed also by Soret at a still earlier date (1875) as I have 

 subsequently found. 



