August 24, 1893] 



NA TURE 



391 



to be paired by a true cock ostrich. This particular ostrich 

 was a hen, although she had every appearance of being a cock. 

 What explanatii n could you give as regards this incoiitniity ? 



"(3) About six months ayo I found a peculiar bird's nest 

 suspended from the root of a mimosa tree which overlapped 

 a bank of ground. Before going further, I must first tell you 

 thai previous to the occasion in question I noticed the same 

 peculiar fonn of nest, but it setmed so utterly impossible at 

 the time that it could be a nest, since its structure and mode 

 of suspension had the exact characteristics of a certain 

 structural spider's web, that I passed it by. But on the second 

 occasion, to make absolutely sure that I had not made a mistake, 

 I went up and cut the nrst off, wiih a certain lergth of the root 

 to which it was attached. Imagine my surprise, when I 

 saw that it was really a bird's nest with two eggs. Now this 

 nest was a ftrfcct facsimile of a common spider's web and 

 home, found in the locality where I was at the time staying. 



" Since it was a marvellous imitation of an insect's habitat, 

 there must have been some corresponding necessity for such 

 imitation. Either the nest must have been designed and con- 

 structed, so as to delucfe enemies by which the species was 

 liable to be aitacked, or, it was so imitated, that the materials of 

 which the nest was made should Ferve as a bait, and allow the 

 parent birds to be able to feed their young without the necessity 

 of having to leave the nest, and so be unable to jrotect their 

 young for the time being. The materials from which the nest 

 was made were practically webs abandoned by their original 

 owners. It was aa instance of perfect imitation." 



Astronomical Photography. 



The announcement (Nature, August lo), that it is in con- 

 templation to raise a sum exceeding £1000 for the establish- 

 metit of a special photographic telescope at the Cambridge 

 Observatory, leads me to ask whether astronomers have duly 

 considered the facllilies afforded hy modern photography. At the 

 time of my eirly experience of the art, thirty-five years ago, it 

 would have been thought a great feat to photograph the Fraun- 

 hofer lines in the yellow or red regions of the spectrum, although 

 even then the statement so commonly made that chemical acti- 

 vity was limited to the blue and ullra-blue rays was quite un- 

 warranted. With the earlier photographic proce.sses the distinc- 

 tion wa« necessary between telescopes lo be used with the eye or 

 for photography. In the former case the foial length had to be 

 a minimum for the yelbw rays, in the latter for the blue rays of 

 the spectrum. 



Hut the situation is entirely changed. There is now no diffi- 

 culty in preparing plates sensitive 10 all parts of the spectrum, 

 witness the beautiful photographs of Rowland and Higgs. I 

 have myself used "orthochromatic " plates in experiments when 

 it was desirable to work with the same rays as most influence the 

 eye. The interference bands of sodium light may be photo- 

 graphed with the utmost facility on plates sensitised in a bath 

 containing cyanin. 



The quesiion that I wish to ask is whether the lime has not 

 come to accommodate the photographic plates to the ttlepcopes, 

 rather than the telescopes to the plates. It is possible that 

 plates already in the market may not exactly meet the require- 

 ments of the case, but I feel sure that a tiiheof the sums lavished 

 upon instruments would put us in possession of plates suitable 

 for object glasses that have been designed for visual purposes. 

 There would be no ditJiculiy even in studying the requirements 

 of a particular instrument, over or under corrected as the case 

 might be. 



A doitbt may arise whether plates jo adjusted would be | 

 ..s sensitive as those now in u.'e. Probably Captain Abney, or 

 iome other authority, could give the required informaiion. For 

 some astronomical purposes a moderate loss of fensiliveness 

 conid hardly be of much consequence; for others doubtless it 

 would be a 'erious matter. Rayi.eigh. 



Terling Place, Witham, August 15. 



The Discussion on Quaternions. 

 I HAVE followed with much interest the discussion on 

 quaternions which has with more or less intermission been 

 I go'ng on in Nature for a long time. 



It has always appeared to me that the student of physical 

 cience would better employ his time by studying the "Ausdeh- 



NO. 1243, VOL. 48] 



nurgslehre" to which some of your correspondents have refei red 

 than by studjing quaternions. 



The wcnderlul work of Grassman is contained in a moderate- 

 sized book in remarkable conliast lo the two terrific volumes of 

 Hamilton, which even Prof. Tait admits that he has not read 

 entirely. The fact that the auidtbntingsjehre could be mastered 

 in a mere fraction of the time that weuld have to be devoted 

 to the mastery of quaternions, is not howe\er theimjoilant 

 point. 



The ausdehnungslehre seems to afford a sjmbolism more fitted 

 for the expression of many recondite conceptions in physics, 

 than anything which quaternions has to offer. Even the 

 "Nabla" does not insinuate itself into Nature's secrets more 

 cunningly than does the "Inneres Produkt." 



Perhaps I may give an instance, which if elementary viill at 

 all events illustrate the extraordinary diiectness with whith the 

 different kinds of "product" reach the heart of a physical 

 conception. 



'Ihink of a mechanical system of any kind which possesses 

 buta single degree of freedom, think of any system of forces what- 

 ever applied to that system, and consider the question of equili- 

 brium. The possible movements of the system loim twists about 

 one screw chain, the system of forces form a wrench upon another 

 screw chain. Equilibrium will subsist if, and only if, the 

 " Inneres Produkt " of the two screw chains is zero. Suppose 

 any system whatever possessing « degrees of lieedom. Dynamics 

 teaches that mniually destructive twist velocities can be im- 

 parted to any n -f i screw chains about whieh the system can 

 twist. Does any conceivable symbolism assign tl ose twist 

 velocities moie leautifi lly than the ausdehnung.-lehre ? Each 

 twist velocity is the " Kombinatorisches Produkt " of all tie 

 screw chains to which it does not coriespond. 



The aptitude of other conceptions ol this grand calculus for 

 physical problems could be as readily exemplified. But I for- 

 bear. Why has not some one ere this tianslaied into English 

 " Die Ausdehnungslehre von Hermann Grassman " iivo, pp. 388, 

 Berlin 1S62 ? KoBERT S. Ball. 



Obseivatoiy, Cambridge, August 18. 



A Curious Optical Phenomenon. 



Dr. I.auder Brunton has asked me to give you an account 

 of a very curicus phenomenon witnessed from the top of 

 Gausta mountain (height 6000 Norwegian feet) in Telemarken, 

 south of Norway. 



We were a party of two ladies and three gentlemen on the 

 summit of this mountain on August 4. 



On the morning of that day the sky was passably clear ; at 

 noon there was a thick fog. Between six and seven o'clock in 

 the afternoon (the wind being south to south-west) the fog 

 suddenly cleared in places so that we could see the suriounding 

 country in sunshine through the rifis. We mounted to the 

 flagstaff in order to obtain a better view of the scenery, and 

 there we at once observed in the fog, in an easterly direction, a 

 double rainbow forming a complete circle and seeming to be 

 20 to 30 feet distant from us. In the middle of this we all 

 appeared as black, erect, and nearly life-size silhouettes. The 

 outlines of the silhouettes were so sharp that we could easily 

 recognise thefigures of each other, and every movement was re- 

 pro< uced. The head of each individual appeared to occupy the 

 centre of the circle, and each of us seemed to be standing on the 

 inner periphery of the rainbow. We eitimated the inner radius 

 of the circle lo be 6 feet. 



This phenomenon lasted several minutes, disappearing wjih 

 the fogbank, to be reproduced in new fog three or four limes 

 but each time more indistinctly. 



The sunshine during the phenomenon seemed to us to be 

 unusually bright. 



Mr. Kielland-Torkildsen, president of the Teleroatken 

 Touiist Club, writes to me that the builder of the hut on tie 

 top of Gau.aa has twice seen spectacles ol this kind, but in each 

 case it was only the outline ol the moimtain that was reflected 

 on the fog. He had never seen his own image, and he does not 

 niention circular or other rainbows. A. Willf. 



ChrisLiania, Augnst 15. 



Supposed Suicide of a Rattlesnake. 

 The letter of Mr. E. .S. Holden, of the Lick Observatoiy. 

 in your issue for August 10, describing how a rattlesnake slruik 



