392 



NATURE 



{Sept. 7, 1876 



by no means the only culprit — is that, even amongst 

 scientific men, botany has come to be regarded in this 

 country as scarcely a serious branch of science at all, and 

 as little more than a kind of biological equivalent of the 

 *■' Use of the Globes," suitable, indeed, for ladies' schools, 

 and useful, also, like " Materia Medica," for the purpose 

 of occupying the attention of medical students so as to 

 keep them out of mischief during their first summer 

 session. 



LETTERS TO THE EDITOR 



[T%e 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. 

 No notice is taken of anonymous communications. \ 



Visual Phenomena 



In support of Mr. Arnulph Mallock's conclusion (vol. xiv. 

 p. 350) as to the cause of the star-shaped appearance presented 

 to the naked eye by a bright point, perhaps the following epitome 

 of some notes which I made last January may have interest : — 



1. Looking at a distant lamp, with both eyes, I see a radiant 

 corona round the lamp. 



2. I find that this corona is composed of two coronae super- 

 posed, one due to the right eye, the other to the left. 



3. Each corona has distinctive features of its own, which are 

 recognised in every observation and have remained the same for 

 years. 



4. The radiant beams (which are the conspicuous feature) are 

 not exactly radial, but are forked once or twice. 



5. The corona is bounded by a peripheral fringe of blue suc- 

 ceeding to red. 



6. The diameter of the corona varies with the diameter of the 

 pupil. The distal portions of the radiant beams are concealed 

 or revealed by the contraction or dilatation of the pupil. 



7. Any given part of the radiance may be cut off by advancing 

 an opaque body in front of the eye, from the same side as the 

 given part of the radiance. 



An obstacle which cuts ofi only the central rays of the enter- 

 ing pencil only dims the central- image, but does not affect the 

 radiance. 



The radiance, therefore, is due to the outside rays of the enter- 

 ing pencil becoming excessively refracted so as to be thrown 

 across the visual line before reaching the retina. 



8. Tliere is nothing in the cornea, aqueous humour, or pupil- 

 lary margin of the ins, that can cause such refraction. 



9. It appears most likely that this excessive refraction of the 

 outside rays of the entering pencil is caused by something in the 

 crystalline lens — probably, in the first place, by undue convexity 

 of the more marginal parts of the lens, which are uncovered by 

 dilatation of the pupil. 



10. Tiie radiant appearance of the corona is probably due to 

 the radiate structure of the crystalline lens. 



The agreement between them is pretty close, extending to the 

 furcate character of the rays. 



11. On examining my own eyes by a pencil of diverging rays 

 (admitted so as to throw a shadow of anything inside the eye 

 upon the retina) I find that the main beams of the radiance cor- 

 respond pretty well to the main radii of my lenses, in the oppo- 

 site direction. 



This confirms that the radiance is due to the radiate structure 

 of the lens. 



In one particular my observations differ from Mr. Mallock's. 

 He finds that the length of the rays depends " on the brightness 

 of the point " as well as on the size of the pupil. I find that 

 points of different degrees of brightness have rays of the same 

 length, that length being limited only by the size of the pupil, 

 but that the breadth of each ray varies with the apparent size of 

 the lamp-flame or other luminous point under observation. In- 

 deed, if the lamp-flame is so bright as to stimulate the iris to 

 contraction, the rays become shortened in like measure. 



The radiate structure of the lens of the human eye is well 

 shown in KblUker's " Manual of Human Microscopic Anatomy" 

 (ed. i860, p. 568). The "non-fibrillated" or "central planes" 

 there described are possibly of greater refractive power than the 



wedge-shaped fibrillated portions between them. This would 

 cause the phenomenon of radiance by excessive refraction of the 

 outside rays of the entering pencil. Hubert Airy 



Edensor, Kidbrook Grove, Blackheath, Aug. 28 



Species andjVarieties 



It may be taken, I presume, that the description and naming 

 of " species" has now a great value as material for studying the 

 laws of the evolution of species and of geographichal distribution 

 and variation ; and that the question is not so much to know 

 what name to call a "species" as to account for its presence 

 and form in the economy of nature. And it will, perhaps, 

 also be granted that the study of geographical distribution does 

 not consist alone in acquiring a knowledge of the fauna of a 

 district, So much as in investigating the laws of the special dif- 

 ferentiation of that fauna. It thus becomes evident that the 

 slightest modification tending to persistency requires the most 

 careful record, as it is only by the knowledge of first slight 

 modifications that we can expect to understand the process of 

 lirger divergencies. 



In faunistic catalogues, should the author be inclined to the 

 " lumping method " (perhaps in one sense correctly), a form may 

 be considered as only a variety of some well-known species, and 

 recorded under that name. We thus learn nothing of its modi- 

 fication, and are led to think of it as agreeing with the typical 

 form, thus losing one of the most important facts in our study 

 of variation and distribution. Another element of error seems 

 also co-existent. When a newly-discovered form is designated 

 a variety of another species, it would lead us to suppose that 

 that species is the original type from which the other form has 

 branched. But it is quite possible to suppose on such reasoning 

 that the newly-discovered form may have been the parent from 

 which the previously described species may have been derived. 

 The offspring may have been collected and described before the 

 parent ; or we may be filling a space with planets to which there 

 is no sun. This I have frequently suspected in the study of 

 exotic lepidoptera, a study wfiich is principally confined to the 

 perfect form of the insects and of whose larval changes we know 

 ill most cases nothing. It rests generally on the judgment of 

 individual entomologists as to the amount of difference to be 

 considered specific. 



Without venturing on the vexed question as to species and 

 varieties, would it not be furthering the cause of science that 

 what are considered as merely local varieties should, as such, 

 have as careful a description as though they were ranked as 

 species. W. L. Distant 



Buccleuch Road, West Dulwich. 



Antedated Books 



I CANNOT allow that our Transactions are antedated, as is 

 aoserted by " Another F.Z.S." As is the case with the /%//«?- 

 sophicul Transactions of the Royal Society and the Linnean 

 Transactions, no date beyond the year of issue is given on the 

 cover, but there is the additional advaatage of the date at which 

 each number goes through the press at the bottom of the sheets. 

 There can, however, I think, be no objection to giving a still 

 more exact date of publication on the cover, and this, in accord- 

 ance with " Another F.Z.S. 's" suggestion, for which I am much 

 obHged to him, I propose to do in future. 



P. L. SCLATER 



I AM sorry to see that " F.Z.S." still prefers to write under 

 the signature of his first letter. I was in hopes that when he. 

 saw the gravity of the accusation which he was bringing against 

 me, he would in his own person have disclaimed any intention 

 of seriously laying such a charge to my door, but as no word of 

 spology escapes him, I regret that in making such an injurious 

 statement he has not added the weight of his name to the accu- 

 sation. I am, therefore, at liberty to suppose that he is no 

 stranger himself to the "evil practice" {honi soil qui maly pense) 

 which he deprecates, otherwise I cannot imagine'such a thought 

 occurring to anyone ; but it is evident that on receiving the 

 book his only impulse was to write a letter to this journal instead 

 of cliaritably endeavouring to discover some feasible explanation 

 of what he calls the " false " date. As most naturahsts in England 

 are in the habit of following the rules of the British Association 

 in regard to nomenclature, "F.Z.S." can find more profitable 



