474 



NATURE 



[Sept. 28, 1876 



There is a curious error in Mr. Wallace's address which 

 seems to deserve notice, as otherwise it will be often quoted 

 without suspicion. He remarks (Nature, vol. xiv. p. 407) : — 



" It must also be remembered, as Mr. Moseley has suggested 

 to me, that a flower which had acquired a brilliant colour to 

 attract insects might, on transference to another country, and 

 becoming so modified as to be capable of self-fertilisation, retain 

 the coloured petals for an indefinite period. Such is probably 

 the explanation of the Pelargonium of Kerguelen's Land, which 

 forms masses of bright colour near the shore during the flowering 

 season, while most of the other plants of the island have colour- 

 less flowers in accordance with the almost total absence of 

 winged insects." 



Now the difficulty is that there is no such Felargonium ^ in 

 Kerguelen's Land, though it is true that the insects are apterous. 

 The flora of Kerguelen's Land is enumerated in the Journal of 

 the Linnean Society, xiv. pp. 389, 390. Of such a persistence 

 as tliat alluded to by Mr. Wallace there is an instance in the 

 case of Prin^lea, of which Mr. Eaton detected some specimens 

 v;ith petals, though the coloration was, I believe very faint. 



W. Thiselton Dyer 



Zittel's Palaeontology 



In the review of Prof. Zittel's " Handbook of Palaeontology," 

 which appeared in Nature, vol. xiv. p. 445, it should have been 

 stated, in connection with the occurrence of Radiolaria in pre- 

 Tertiary beds, that Mr. W. J. Sollas, of Cambridge, met with 

 specimens in coprolites from the Upper Greensand, some three 

 or four years ago, vide Geol. Mag., 1873, vol. x. p. 272. Prof. 

 Zittel duly records this fact in his paper on *' Radiolaria from the 

 North German Chalk," and mention of it was accidentally 

 omitted in the concluding paragraphs of the review. 



Newcastle-on-Tyne, September 25 Henry B. Brady 



Visual Phenomena 



It is evident A. Mallock (vol. xiv. p. 351) and H. Airy 

 (p. 392) describe tWo different, though allied, phenomena ; the 

 latter describes the appearance of rays of light, which, after 

 entering the eye, meet at a focus, and diverge before reaching 

 tlie retina ; and the former, that of rays which reach the retina 

 before meeting at a focus. For convenience, I shall call these 

 respectively the "over-refracted radiance" and the "under- 

 refracted." They differ in the following particulars in the case 

 of my own eyes : — 



T/te Over-Refracted Radiance I The Under- Refracted Radiance 



(i) is diminished I is increased 



by concave spectacles. 



(2) is increased ( is diminished 



by convex spectacles. 



(3) any given part may be cut off by advancing an opaque body 



in front of the pupil from the 

 same side as | opposite side to 



the given part of the radiance, 

 is red outside. 



consists chiefly (when small, 

 entirely) of well-defined rays, 

 mostly forked. 



is increased proportionately, or 

 more than proportionately, to 

 the size of the pupil. 



(4) is green outside (or blue, 

 if sunlight is used). 



(5) consists chiefly of a more 

 or less perfect ring surrounding 

 indefinite rays. When the ra- 

 diance is very large the rays 

 disappear in the general bright- 

 ness. 



(6) is not materially increased 

 by increasing the size of the 

 pupil ; unless the radiance is 

 very large, and even then it is 

 increased much less than in pro- 

 portion to the pupil. 



The first four of these must evidently, from the theory of the 

 mode of production of the two kinds of radiance, be constant for 

 all eyes ; but not knowing the cause of the last two, I cannot 

 say whether they are so, or whether they are peculiar to some 

 ^y^s. From H. Airy's notes, 4 and 6, it would appear that 

 the latter is the case ; for he is evidently short-sighted. 



Though contracting my pupil to its smallest size has little effect 



f Thtre is z Petarg-onium in Tristan d'Acunha (sea Moseley, in %«;». 

 Limi. Soc. XIV, p. 383.) it J ■ 



in reducing the over-refracted radiance, yet by placing still 

 smaller diaphragms in front, I can reduce it almost to a point. 



With my naked eye I see both kinds of radiance ; No. 3 of the 

 above differences supplying an easy means of sepirating them, 

 viz., by covering half the pupil. All then that lies on the same 

 side as the exposed part of the pupil, belongs to the over-refracted 

 radiance ; and that on the other side to the under-refracted. But 

 as I am rather short-sighted, the over-refracted radiance (which 

 makes a nearly octagonal corona) greatly preponderates ; so that 

 I was not previously aware that the other existed with my naked 

 eye. The application of the weakest spectacles (convex or con- 

 cave) completely abolishes one or the other. 



A. Mallock is hardly correct in calling his " phenomenon A " 

 a limiting form ; he is probably what is often called ' ' long- 

 sighted " — I do not know whether there is any proper name for 

 this peculiarity of vision — for the limiting form of both kinds of 

 radiance is a point. This is what I. W. Ward sees (see p. 423), 

 his eyes being neither too refractive nor too little refractive, but 

 just right (he uses the word " long-sighted " in a different sense 

 from what I have). As most people are either long or short- 

 sighted, they see one or the other kind of radiance with the 

 naked eye ; but it also appears from my own case that a person 

 may see both kinds together, and such cannot see a point of 

 light free from radiance, no matter what spectacles he uses, 

 unless he uses a diaphragm. I should be interested in hearing 

 whether, when I. W. Ward looks through spectacles, the rays 

 appear. 



The brightness of the point scarcely affects the size of either 

 kind of radiance ; but a red glass between the eye and it cuts 

 off the outer edge of the over-refracted radiance. 



Unlike H. Airy, I have failed to discern any relation between 

 the positions of the beams of the over-refracted and under- 

 refracted radiances, except that I suspect that the beams in the 

 one may in some degree correspond to the opposite gaps in the 

 other. 



It would appear from the foregoing description of the phe- 

 nomena in question more probable that the "wedge-shaped" 

 portions of the crystalline lens, alluded to by H. Airy, instead 

 of having the least refractive power, as he suggests, really have 

 the greatest. The question arises, do different eyes differ in this 

 respect? Thos. Wm. Backhouse 



Sunderland, September 19 



Antedated Books 



The writers who have called attention to this point in Nature 

 will have rendered an important service to students if they obtain 

 an amendment in the lax system often followed in this country. 

 But it is to be hoped that the reform may be made complete. It 

 is perfectly easy to have the exact date of issue and the number 

 and letters of the sheets contained in the part stamped on the 

 wrapper or cover, and then by binding these wrappers with the 

 parts, an exact reference to the date is always at hand. This ii 

 done with praiseworthy exactness by some of the foreign societies. 

 For instance, I receive this morning a part of the Annales de la 

 Soc. Entofuologique Belgiqtie, the wrapper of which bears "Tome 

 dixneuvieme, fasc. i., signatures i a 13 et « a /! Paru le 16 

 Septembre, 1876." This it will be seen leaves nothing to be 

 desired. Even in Spain, which we flatter ourselves is so far 

 behind us, they manage this point accurately. The Annales de 

 la Soc. Esp. de Hist. Nat. bears on the wrapper of each cuaderno 

 the exact day of publication. Now that attention has been called 

 to this point, let us hope that each society will instruct its secretary 

 or editor, to see that the exact day of publication, and a summary 

 of its content;, be stamped on the wrapper of every part issued. 



Thornhill, September 19 D. Sharp 



OUR ASTRONOMICAL COLUMN 



The Binary Star ^ Bootis. — A satisfactory orbit for 

 this fine star is still wanting. Elements founded upon 

 measures to 1833 were calculated by Sir John Herschel 

 (period 117 years) ; and an orbit, on an extension of mea- 

 sures to 1854, was given by Hind (period 169 years), but 

 later observation has shown them to be madmissible. 

 The great difficulty encountered in this case undoubtedly 

 arises from the errors which must exist in one or more of 

 Sir W. Herschel's angles, as was pointed out by his son 

 in the Memoirs of the Royal Astronomical Society, vol. vi,, 



