284 



NATURE 



\yuly 28, 1 88 1 



are chromolitho^raphed from the author's own drawings, 

 which appear to be exceedingly well done. We defer a 

 more extended notice until more parts shall have appeared, 

 especially because the subjects illustrated in the first part 

 are almost the least difficult for the chromolithographic 

 process. The te^t is clearly printed, but a little more 

 care in writing the short descriptions should be exercised. 

 Thus at the very commencement we read as one of the 

 characters of the family Papilionida;, " Larva cylindrical, 

 not spiny, furnished with two retractile tentacles on the 

 second segment." We doubt if this is correct for all the 

 European species of Papilioj it certainly is not so if 

 exotic species of the same genus are considered ; and 

 almost immediately afterwards the author, in defining the 

 genus Tluiis (one of the Papilionidiv), says, '' Larva? 

 armed with spines." Nowhere do we find any reference 

 to the veining of the wings, which certainly should have 

 formed part of the sketch of the principal groups given 

 in the Introduction. The author will do well to consider 

 the importance of this suggestion. We presume the chief 

 object of the work is to enable collectors of European 

 butterflies to name their captures, and especially by 

 means of the figures. For this purpose it promises to be 

 exceedingly well adapted. 



LETTERS TO THE EDITOR 



[ The Editor does ttot hold h iinself responsible for opinions expressed 

 by his conrspottdettts . Neither can he undertake to return, 

 or to correspond with the writers of, rejected manusiripts. 

 A''o notice is taken of anonymous commtmicatiotis. 



[ 7721" Editor urgently requests correspondents to keep their letters 

 as short as possible. The pressure on his space is so great 

 that it is impossible othe>~wise to ensure the appearance ev:n 

 of communications containing interesting and ncrel facts.\ 



Chemical Equivalents 



Mr. J. P. O'Reilly's paper in your la^t number (p. 274) 

 appears to involve a coTiplete misconception of the theory of 

 chemical equivalents. The equivalents are mere ratios, aud are 

 not altered by multiplying their representative numbers through- 

 out by any factor, whether tt or any other. 



In fact we may write the equivalents of hydrog;en, carbon, 

 and oxygen as x, 12, .v, and \tx, without troubling ourselves 

 about the value of -v. This is not only the theoretical view, but 

 the one actually u^ed in practice. So far, there is nothing new 

 or special about writing, as Mr. O'Reilly does, .r = \i!. 



But then Mr. O'Reilly goes wrong, and gets results which 

 contradict his hypothe^is. When he writes H = Jtt, and O = 5 tt, 

 the real inference is tliat the equivalents of H and O are as I : 15, 

 i'lstead of as 1 : 16 which he started from. 



If his ir-values are to be taken as corresponding with the equi- 

 valents, this simply means that the latter are not to be depended 

 upon within a limit of error of 5 per cent. I think the mistake 

 is not in this respect, but in overlooking the cu-cumstance that 

 the chemical equivalents are not absolute values, but ratios. 



July 24 C. W. M. 



Slow Lightning 



Having just seen the statement of Prof. Tait (Nature, 

 vol. xxii. p. 341) quoted, as a final authority, against the possi- 

 bility of distingui>lnng the source from the termination of a 

 lightning flash, I wish to record a storm that I saw. On May 19 

 there had been a bris!;, hot south-west wind blowing at Gizeh, 

 off the Libyan Desert, at about or over 100° F. ; at near sunset 

 a north wind began to come up against it, and there was 

 heavy thunder and lightning all along the line of the mingling 

 of the winds, extending as far as I could see to east and west, 

 and passing a few miles to the north of the Pyramids : the light- 

 ning was solely between the clouds, at a hei;^ht of about one and 

 a half miles ; the air around me was 94°, though almost dark. 

 I sat on a rock in front of the door of my tomb (from which I 

 could see eighteen miles over the Delta) and quietly watched the 

 lightning. To my sight there were distinctly differences in the 

 duration o! the flashes : some appearing instantaneous and others 

 in which I could see a spot of light occupying an appreciable 

 interval to travel from one cloud to another ; and I should be 

 puzzled to draw a hard and fast line between the classes. Does 



this moving spot-lightning merge insensibly into the variation, 

 of which I saw a fine case years ago near Guildford, where a 

 spark would slowly sail down in the air and then move over the 

 ground before it disappeared ? 



In any case can these slow flashes (lasting perhaps half a 

 second), seen as well as instantaneous flashes, be disposed of by 

 that blessed word sidijectivity, which is so comforting to theorists 

 on many objects ? Or may not the confession of our ignorance 

 of the cause of ball-lightning be extended to slow flashes in 

 general, instead of treating them just as meteorites were put out 

 of court a century ago ? W. M. Flinders Petrie 



Bromley, Kent 



[Several instances are recorded by Faraday, Joule, and others 

 of flashes which seemed to last for a sensible time. But they 

 are easily explained by one or other of two verce causir, viz. (i) 

 oscillatory discharges along the same path, succeeding one 

 another at smaller intervals than one- seventh of a second ; or (2) 

 phosphorescent matter in tlie track of the flash. More definite 

 particulars would be necessary before one could decide which 

 was active in the present case. — Ed.] 



Thought-Reading 



As having a bearing upon the hypotliesis that in "thought- 

 reading " the information is transmitted by unconscious muscular 

 exertion, allow me to state a modified forai of the experiment I 

 tried in the presence of two or three others with Mr. J. R. 

 Brown, who, a few years ago, attracted considerable attention in 

 various parts of the United States by doing precisely what is 

 related of Mr. Bishop in your issue of June 23 (p. 171). Aft^r 

 witnessing experiments of the same kind as those stated by Mr. 

 Romanes and performed under the same conditions, I thought 

 to vary them by using a flexible copper wire as a connecting 

 medium. Selecting one, two or three yards long, I held one end 

 in my hand, while Mr. Brown, winding the other end once or 

 twice around his fingers, held it against his forehead, the wire 

 being all the time kept slack between us. Here evidently there 

 could have been no indications received through muscular move- 

 ments. Yet in this way Mr. Brown would find things concealed 

 or go to certain points determined upon, though apparently with 

 not quite the same readiness and confidence as when the subject's 

 hand was placed against his forehead. Once he partially failed, 

 selecting, instead of a particular spot on the wall I had fixed my 

 mind upon, a small object near it. The experiment in this form 

 was tried with another as his subject, and with equal, if not 

 better, success. Geo. B. Merriman 



Rutger's College, New Jersey, July 1 1 



Optical Phenomena 



The photographic halo phenomena described in Nature, vol. 

 xxiv. p. 260, seem analogous to some observed by me, and upon 

 which, in the spring, I read a paper (since published in the Notices, 

 vol. xli. No. 6) before the Royal Astronomical Society. In this 

 I described that not only the sun's disk and the moon's full and 

 partial phases, but also apertures (of similar shape to these) in 

 the shutter of a dirk room, when photographed, were, one and 

 all, surrounded by a strong ring halo not visible to the eye. A 

 correspondent essayed some time since to prove in your journal 

 that this halo only surrounded the moon when at full, but on 

 trial the question proved one of time of exposure ; and it now 

 seems pretty clear that whatever may be its form and nature, a 

 very bright object when photographed (especially in relief 

 against a dark ground) is found, if sufficient exposure be given, 

 suiTounded on the plate by a halo separated from the object by 

 a dark space. Mr. Cowper Ranyard and others attribute these 

 halos to reflection from the back of the plate, a point on which 

 I have not experimented. The dark spot mentioned in connec- 

 tion with the aperture in the rock is probably a reversal of the 

 brightest light owing to the length of exposure. 



fn two seconds, with Wratten and Wainwright's instantaneous 

 plates, I have found the sun's image so reversed in a camera 

 landscape, showing as a v\ hite spot on the negative and a black 

 dot when printed. J. Rand Capron 



Guildown, July 23 



Symbolic Logic 



Mr. McColl still expresses surprise at my declining to answei" 

 a Yes or No question which he was pleased to put to me in 



