November io, 1892] 



NATURE 



31 



Optical Illusions. 



Referring to the article in Nature for October 20, may I 

 aiention a rather common optical illusion which I do not re- 

 member to have yet seen in print. If a gothic arch is unequally 

 divided by a space between two vertical parallel lines, these 

 lines will not only seem to diverge slightly where they intersect 

 the lines of the arch, but the arch itself is caused to appear as if 

 one half had slipped bodily down from the other to an extent 



equal to its own thickness. In the figure given above it is 

 impossible to believe that but for the intervention of the vertical 

 interlinear space the two halves would be seen to meet perfectly 

 with the apex at A. This illusion is worth the notice of architects 

 who desire to avoid the disquieting effect upon the eyes of 

 observant persons which is produced by the intersection of the 

 chancel arch of a church by an intervening pillar. 

 28 Mount Park Crescent, Ealing, W. R. T. Lewis. 



A Remarkable Rainfall. 



The rainfall here of October has been so remarkable that it 

 seems worth While to place it on record in your columns. Rain 

 fell on twenty-five days during the month, making a total fall of 

 10-32 inches. As the annual rainfall on an average of eleven 

 years is 31 -lo inches, it will be seen that very nearly one-third of 

 this amount fell in one month. This is by far the highest amount 

 I have recorded since I began to make records in January, 1878, 

 the next highest month being August, 1879. On that occasion 

 five inches fell in thirty hours on the 17th and i8th, and many 

 bridges were carried away in Flintshire and Denbighshire, but 

 the total fall for the month was only 7*89 inches. Dr. Nicol, of 

 Llandudno (six miles from here), who has registered the rainfall 

 since, and including 1861, informs me that it amounted last month 

 to 8*56 inches there, this also being the highest month he has 

 ever recorded. 



In September rain fell on twenty-three days, and though the 

 total fall was only 377 inches, yet the constant rain, combined 

 with an unusually low temperature (the mean maximum being 

 only 56°'6, and the highest shade temperature b"}"'^, against 

 64°-6and 8i°-2 respectively in 1891), made it almost impossible 

 to get in the harvest. Alfred O. Walker. 



Nant-y-Glyn, Colwyn Bay, November 5. 



On a Supposed New Species of Earthw^orm and on the 

 Nomenclature of Earthworms. 

 In yesterday's Nature I find that the Rev. Hilderic Friend 

 has again given the name Z. rubescens (Friend) to a supposed 

 new species of earthworm. This worm appears to me to be 

 identical with Enterion festivum (Savigny), described under 

 the name Lumbricus festivus by Rosa. Though comparatively 

 rare, it is by no means new, nor even new to Britain, though I 

 know of no published record of its occurrence here. I met with 

 two or three specimens among the worms supplied to me when 

 I was working at the chapter on Lumbricus in " Marshall and 

 Hurst," and identified them subsequently by the help of Rosa's 

 lable. At the time I took them for mere varieties, and put them 



NO. 1202, VOL. 47] 



into a bottle for future study. I believe the specimens are now 

 in the possession of Dr. Benham, who has entirely overlooked 

 the species in his "Attempt to Classify Earthworms " (^warA 

 Joiirn. Micr. Set. xxxi.). 



The specific name terrestris must also, it appears to me, be 

 dropped. Linnaeus did not define a species under that name, 

 but applied it to what are now universally regarded as several 

 distinct species. The species so called by Mr. Friend was, I 

 believe, first defined by Savigny under the name Enterion 

 hercnkitm. The diagnostic characters of the species are given 

 by Rosa in his useful table of the species (" I Lumbricidi dei 

 Piemonte," p. 25), and he calls it Lumbricus herculeus, to which 

 name the usual rules of nomenclature bind us. 



I would therefore suggest the following alterations in Mr. 

 Friend's "Chart of the Genus Lumbricus" :— 



1. For " Terrestris (Linn.) " read " herculeus (Sav.)" 



2. For "Rubescens (Friend)" read "festivus (Sav.)" 

 Owens College, October 28. C. Herbert Hurst. 



Ice Crystals. 



During the cutting of the formation for a railway I observed 

 on Tuesday morning, the i8th inst., a peculiar series of ice 

 crystals. The ground is composed of arenaceous clay largely 

 mixed with sand and small gravel, and is of a very open nature, 

 the surface being covered with moorland grass, rushes, and 

 coarse ferns. These crystals were only found in a length of 

 about nine feet, the ground on both sides of the patch being 

 hard frozen. 



These crystals were acicular, and sprang from a base of very 

 porous opaque ice, but every needle was entirely free and dis 

 tinct throughout its height, and at first sight appeared to be 

 bound together with two bands, one at one-third and the other 

 at two-thirds of the height. A closer examination proved that 

 the band appearance was due to a slight enlargement of the 

 crystals at these points, the ice being opaque, whilst the 

 needles were perfectly translucent. 



The average height of these crystals was about one inch, the 

 needles having a diameter of about ^Vth part of an inch, and were 

 grouped together in clusters of forty or fifty, forming an irregular 

 square of about J-inch on the side. Some of these crystals were 

 growing vertically from the ground, others springing out hori- 

 zontally from the side of the cutting, and were either straight, 

 curved, or bent round foriiing a half circle. This morning the 

 same form of crystals existed, but were much larger, being fully 

 two inches long. On both occasions the air was calm and clear, 

 the min. then reading 30° on the i8th, and 24° to-day. 



Lesmahagow, October 25. C. M, Irvine. 



Lunar Craters. 



The letter and illustration offering a suggestion as to the 

 formation of lunar craters remind me of an experiment I once 

 saw during a chemical lecture, bringing out the same point very 

 clearly. 



A shallow dish containing a layer of damp sand, V, was flooded 

 with I -inch coating of Paris plaister, of the consistence of cream, 

 and the dish set to dry over a Bunsen flame. 



As the plaister set, the surface was pitted with crater-like holes, 

 formed by the escape of steam from the sand at the bottom of 

 the dish, giving a perfect representation of a lunar surface. 



As some of your readers might care to try this experiment, I 

 take the liberty of sending you this "recollection." 



M. H. Maw. 



Walk House, Barrow-on-Humber, Hull, Nov. 7. 



A Fork-tailed Petrel. 



The occurrence of a Fork-tailed Petrel as far inland as 

 Macclesfield may perhaps interest some of the readers of 

 Nature. 



It was picked up by a man on the nth ult., two days after 

 the stranding of the Sircne in a gale at Blackpool, and being 

 unacquainted with the species he sent it to me as a curiosity. I 

 identified it as a Fork-tailed Petrel, and Mr. J. H. Salter, of 

 Aberystwyth College, has kindly confirmed this decision. 



Some of the feathers on the forehead are tipped with white. 

 Does this indicate a young bird, as I can find no mention of it 

 in any of the plumage descriptions that I have seen ? 



Newman Neave. 



Rainow, near Macclesfield, November 5. 



