484 



NATURE 



[February 8, 191 



Are Eyes ever Autophanoufl ? 

 I VENTURE to suggest the following simple explanation 

 of the phenomenon described by Colonel Herschcl in 

 Nature of January 18. At the distance at wh.ch his 

 experiments were made the light from the lantern or other 

 source enters the lens practically as a pencil of parallel 

 ravs, which is concentrated as a bright spot on the retina. 

 The divergent light from this bright spot, which passes 

 backwards through the lens, is again made into an almost 

 parallel pencil. If the retina is exactly in the focal plan.- 

 of the source of light, and the image an absolutely sharp 

 one the whole of the light is again concentrated on to the 

 buH's^ye of the lantern, and without the use of a trans- 

 parent, but partially reflecting, surface, it must be 

 impossible to see it. 'But the least blurring of the imago, 

 from whatever cause, leads to a slightly divergent pencil, 

 which, however, is still narrow enough to concentrate the 

 returning light within a degree or so from the direction 

 of the original source. To an observer withm this cone 

 of rays the bright spot on the retina will appear to fill a 

 part or the whole of the pupil, just as a black dot may 

 be made to fill the whole aperture of a lens to an observer 

 at a distance by holding the lens at its focal distance from 

 the point in question. 



•\ cat's eyes when in shadow may often be seen by a 

 watcher, himself in the light, to be filled with a faint 

 luminescence, which disappears when the watcher s own 

 eyes are shaded. The source in this case must be the 

 bright surface of the face of the observer, possibly com- 

 bined with the reflected point or points of light on the 

 exiernal surface of the observer's eyeball. The pheno- 

 menon is rather a curious one, and may account for a 

 certain number of the cases in which an animal's eyes are 

 supposed to be autophanous. E. M. Anuekson. 



Edinburgh, January 27. 



Chalk and Ice. 



Two nights of hard black frost, following upon the 

 recent wet weather, has resulted in pieces of chalk resting 

 upon a wet clay soil becoming curiously coated with ice. 

 This occurs only in the case of chalk, other stones— except 

 fragments of brick, which have a thin veneer of ice on 

 them— and lumps of clay being free from it. 



A piece of chalk 2 in. X 2 in. X 1 in. embedded in the 

 soil is covered with ice as in Fig. i. The ice is ftbrous. 

 Small pieces of chalk give forms as in Fig. 2. The central 

 figure evidently illustrates two nights' growth, the upper 



^'^^^^'^^^ 



Fig. L 



beneath the nurfaco. 1 he rising water in the chalk 

 to prevent the latter freezing, the cold spending ts. 

 producing the ice resting upon the chalk. 



It is conceivable that a stratum of stone* and ■ 

 ing upon chalk, might in a severe climate Lm 

 separated from the chalk by an ice layer. Th« fonr 

 of ice below the surface in this way m.. . loi 



the " soil creep " which has occurred in th< i tm 



(jfTects of which are now so noticeable in ch..... 



D. Dkii 

 Inglewood, Longcroft Avenue, Harpenden. 



Glazed Frost. 



TiiK deposit of "glazed frost." sometimes w 

 markable thickness in Newfoundland, and the gr^ 

 of ice formed causes considerable flexure <.f 

 branches of trees. 



I have seen every twig and «very blade ■ . l.ipH 



cated on the windward side in clear ice, the ice " t 

 often having a diameter two, three, or even more 

 that of the twig on which it was formed. 



Thick slabs of ice appear on the windward sides o 

 tree trunks, palings, and even of walls. In Ncwfoun<ilnn. 

 the rain is often followed by bright sunshine, whf 

 effect is most beautiful, and is aptly described in th' 

 name "silver thaw." The explanation of the \ 

 menon, quoted from "The Observers' Handbook" 1 

 Harding's letter to Nature of January 25, would s^ 

 be justified by the thickness of the deposit formed on 

 and on blades of grass; it would be diflficult to ac. -. 

 for a thick deposit of ice if the raindrops were not sup« 

 cooled before touching the trees or ground. 



E. R. Ma! 



Hartley University College, Southampton, 

 January 29. 



The Thawing of Frozen Water Pipes, 



.\s the present frost is causing serious inconveni' 

 manv houses, I should like to direct attention to a n 

 of thawing ice in pipes which I have frequently pi 

 practice and found effective. It is based on the pr 

 that strong brine eats its way into the ice like an ac: 

 that the resulting diluted brine rises and makes rcx 

 the denser fluid. Close the main tap, and with a m 

 unscrew the top of the valve of the frozen water-pii> 

 remove the small valve. Ball taps to cisterns may iia\ 

 to be unscrewed altogether. Insert a few feet of on. 

 eighth inch rubber or lead tubing into the pipe, anj 

 concentrated brine into it through a small funnel. K 

 the valve and cover, but leave the valve open ; al«. 

 the main valve, and wait. If the ice plug in the ; 

 onlv I or 2 feet long it will have been eaten thrc 

 about an hour's time; if longer, the above operation 

 be repeated several times. The brine is prepared b 

 ing an excess of salt in water, say half a pound pt : 

 it should, if possible, be used hot, and renewed freu 



West Didsbury, February v <-"• F- "^tromi 



cone having been thrust off the chalk by the growth of the 

 one below. 



The moisture has evidently frozen on the chalk, fresh 

 moisture rising from the ground and passing through the 

 chalk, thus keeping up the supply. The chalk itself is 

 not frozen, and can be broken easily by hand, but the 

 pillar of fibrous ice is firmly frozen to it. 



We also noticed tooth-like pieces of ice projecting from 

 the lawn, and in every case these were found to rest on 

 pieces of chalk beneath the surface. An examination of 

 the flower beds revealed bosses of ice coming up through 

 the soil, the ice in all cases resting on lumps of chalk 



NO. 2206, VOL. 88] 



The Names of Fossil Plants. 

 Every botanist must agree with Dr. Marie Stopes th: 

 there should be some ready way to distinguish betw'- - '' 

 fossil plants which are referred with reasonable as- 

 to their genera, and those which are merely pin 

 genera which they seem to resemble, but to whic 

 very probably do not belong. We must also agr^ 

 it is unsatisfactory to call every doubtful leaf " Phy 

 without anv serious attempt to indicate its affinities 

 use of Gothic type, su^ested by Dr. Stopes f 

 doubtful genera, has some disadvantages, one v 

 being the inconvenience to the printer. Would it : 

 simpler to use quotation marks, in the case cited by D, 

 Stopes writing " Ophioglossum " granulatum? Tf 

 generic name only should b« enclosed within comma= 

 alone is supposed to be in doubt. The method p- 

 has the advantage of being already more or less cu.- 

 and readily understood bv anv reader. 



T. D. A. COCKEREUUJ 



University of Colorado, Boulder. 



EREixJ 



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