January 31, 1895 J 



NATURE 



321 



Perseid?, but subsequent observations have convinced me thit 

 they hid no relaiion with the great August shower. They prob- 

 ably formed the early mem' ers of a welldelined raihant of 

 September Aurigidi which I found at 76° + 56° in 1879, and at 

 77° + 57' in 1885. 



It appears to me that observations of the Perseids, and of 

 other meteor showers, are often undertaken and discussed while 

 losing sight of a most important circumstance. I refer to the 

 necessity of thorough training on the part of the observer, before 

 he can possibly hope to attain a high degree of precision in re- 

 cording meteor paths. Many month , if not years, of diligent 

 practice are required to render the observer proficient, and even 

 then there are many students who, being deficient in natural 

 aptitude, will never succeed in the work. It seems to be the 

 fashion at certain observatories to set a number of observers 

 (some of whom have perhaps never registered a meteor path 

 before) watching and recording meteors, and then to investigate 

 their results as though they could be thoroughly depended upon. 

 Such results are, however, pr.-ictically useless when emploied to 

 test any complicated point in meteoric astronomy. It is simdar 

 to placing a man, who has never played in a cricket match be- 

 fore, as wicket-keeper to fast bowlers like Mold, Richardson, 

 and Woods, and expect his performance to be creditable ! In 

 meteoric astronomv, as in many other spheres of action, skill is 

 only to he a-quired by long practice ; indeed, it is difficult to 

 single out any other branch of observation where the e\e and the 

 judgment have to be so quickly and accurately brought into play 

 to afford the best results. W. F. Denning. 



Bristol, January 27. 



The Artificial Spectrum Top. 



In the interesting letter; on the above subject, which have 

 recently appeared in Na 1 itre, there does not seem to have 

 been any reference to the experiments of llelmholtz, as de- 

 scribed in his " Handbuch der Physiologischen Optik." 1866, 

 § 23. He describes the facts in minute detail, and illustrates 

 them with numerous diagrams. 



One important point not yet referred to, and described in 

 detail by Helmhollz, is that if a disc, marked with black and 

 white sections, be rotated with a certain rapidity, the field 

 appears to be covered with a pattern composed of hexagonal 

 spots ; at the part of the field of vision corresponding to the 

 yellow spot, a transverse oval figure is seen. In the centre 

 of this figure is a dark spot surrounded by a black circle. 



Each of the hexagonal spots is dark with a lighter spot in the 

 centre, and surrounded by a red thread, which appears to be 

 moving in minute drops. The field seems to be pervaded 

 by a greenish hue, which flows towards the yellow spot. 



These experiments, which I have verified on every point, 

 have a vcy important bearing on the photochemistry of the 

 retina and on colour vision. 



Hendon, January 26. F. W. Edriuge-Green. 



In reference to your Belfast correspondents' interesting 

 experiments with the artificial spectrum, which were long ago 

 included in my own experiments, a little reflection will show 

 that when the speed <pf rotation is increased, we do not retain 

 unaliercci the resultant proportion of stimulus an-i anti-stimuli's 

 on ihe retina. With a slow rotation we hive simultaneously 

 on the retina a persistence image of the lines and a real image 

 of ihe white card. When the speed is greater, we get simul- 

 taneously llu-se two, and in addition a persistence image of the 

 white card. Hence, according to my theoi)', ihe rise in scale 

 wi'h increased rapidity of revolution. 



Colchester, January 26. CHARLES E. Benham. 



Snake Cannibalism. 



The reading of a paragraph and a letter printed in the 

 Mail for October 24 and 29, reminds me of a case of one snake 

 swallowing another, the consequences of which I witnessed. 

 While engaged in running a survey line for a railway across a 

 wood in this district, I noticed a snake close to me, doing 

 its best to get out of my way, but almost unable to do so. One 

 of my men struck at its neck with his " macheti," and suc- 

 ceeded in cutting the snake's head clean off. Immediately, to 

 our great surprise, another snake of the same species slowly 

 emerged head first, and, after a few struggles to escape, remained 



NO. 1318. VOL. 51] 



motionless on the ground ; a gash in its cranium, which had 

 been cut by ihe same stroke that killed the larger snake, being, 

 no doul)t, the cause of death, as the body was otherwise intact. 

 A measuring tape shiiwed that the larger snake was 6 feet in 

 length, and the smaller 5 feet. In this case the snake was 

 swallowed tail first, and therefore it seems highly probable that 

 the larger snake simply devoured it, and did not commence by 

 trying to dispute a portion of food, such as the pigeon and frog 

 cited in oihir instances. H. Tsnagal. 



Sancti Spiritue, Cuba, November 23, 1S94. 



More about Moths. 

 ^^Communicated by ProJ. S. Garman, of the Museum of Com- 

 parative Zoology, Cambridgf, U.S.A.) 



I.N Nature for December 6, 1894 (p. 127), Mr. Henry Cecil 

 publishes a criticism on a previous letter of mine, which I 

 cannot accept without a few words of remonstrance. His 

 explanation may be correct in part, but it certainly does not 

 cover all the ground. 



That resistance alone is not necessary for the expansion of 

 the wings of moths, may be inferred from the fact that they will 

 often expand after an interval of several days, when the moths 

 have been prematurely released, the irregularity in outline 

 arising, I think, from the evaporation of moisture from the 

 wings, and in the consequent loss of elasticity. If the newly- 

 hatched insects are confined in a warm moist box, this trouble 

 seems to be obviated in a large degree, and the wings occasion- 

 ally attain to nearly the normal dimensions. 



In raising moths artificially, it cannot be assumed that the 

 lack of proper pressure is entirely responsible for the frequent 

 occurrence of cripples. 



All the conditions of feeding, moisture, and heat, must first be 

 carefully considered, since departures from the normal, on any 

 one of these line-, might so lower the vitality of the insect, that 

 perfect development would beome impoisible. 



The writer also speaks of the wings of the moth in the cocoon 

 as "folded and crumpled," a statement which is entirely at 

 variance with my own observations. In all the cases which I 

 have noticed, the wings are perfectly smooth and unfolded 

 from the first, the increase in size resulting from a true ex- 

 pansion, the nature of which has, so far as I know, never been 

 fully explained. L. C. JoXES. 



Melrose, Massachusetts. 



THE PHYSICAL SOCfETVS ABSTRACTS OF 



PHYSICAL PAPERS FROM FOREIGN 



SOURCES. 



THE days when learning meant dead languages, and 

 science meant collecting beetles, have passed 

 away ; science has grown and spread until it is impos- 

 sible for the most comprehensive intellect to grasp more 

 than a few twigs on its numerous branches. Organised 

 specialisation has become necessary to scientific progress. 

 Each subject now has its special society, and each society 

 has as much as it can do. Every sort of time-saving 

 arrangement is necessary if the workers in any one 

 branch of knowledge are to be kept informed as to what 

 Others are doing. 



English chemists have long been supplied by the 

 Chemical Society with excellent abstracts of the current 

 literature of their subject, but up to the present the only 

 available work of the same kind on Physics has been the 

 Beiblatter of Wiedcmanris Annalen. .Admirable as 

 these are, it is impossible that a (German periodical can 

 fully meet the wants of Anglo-Saxon physicists. It is 

 therefore most desirable that abstracts of physical 

 papers should be published in English. The Physical 

 Society has now set itself to supply this want, and the 

 first number of the new volume of " .Abstracts "appeared 

 early in the present month. The Proceeilings of the 

 Society will in future be issued monthly, and the abstracts 

 of foreign papers on physics will be included under the 

 same cover. They will, however, be paged separately, 

 so that they can be bound separately at the end of the 



