Jan. 4, 1872] 



NATURE 



silence by Mr. Newcomb ; but it is important. It would be 

 difficult to select from such groups of observers — the French, for 

 example, who saw no connecting ligament — those who saw "the 

 phenomena exactly as we know they are ;" and unless this can 

 be (lone, I am afraid that Mr. Newcomb's somewhat unique ar- 

 gument upon this point might be made as easily to prove the 

 converse as the result he deduces from it. All these observa- 

 tions, in my view, are good ; but they are not strictly observa- 

 tions of the same phenomena. 



Mr. Newcomb rejects at once the force of the evidence of the 

 observers of the transits of Venus, 1761 and 1769, upon the 

 question of the connecting ligament "till we have better 

 evidence than now exists that their object-glasses were such as 

 Clarke or Foucault would call good." The phenomena con- 

 nected with the ligament must be far more marked in the case of 

 A'enus than in that of Mercury, on account of the large diameter 

 of \"enu^.. To reject therefore by an impossible condition all the 

 evidence in our possession respecting transits of Venus is cer- 

 tainly a bold step ; but Mr. Newcomb appears to me to attach 

 far too much importance, so far as irradiation phenomena are 

 concerned, to the improvements effected in modern telescopes. 

 The Image of a point of light on the most perfect object-glass 

 which can be conceived is not a point, but a disc, of which the 

 illumination degrades rapidly from the centre, and which is sur- 

 rounded by concentric rings of light. The law of degradation of 

 the illumination of the central disc has been given by me in the 

 Monthly Notices, November 1S65. The result of theory upon 

 these points has been most completely tested by experiment. 

 The existence and regularity of these concentric circular rings is 

 one of the most delicate tests of the perfection of a telescope. 

 Since we have a disc of light corresponding to a point in the 

 most perfect object-glass which can be made, the visible image 

 of the sun formed by such a glass will not terminate with the 

 geometrical image. This result of theory is confirmed by experi- 

 ment. The optical enlargement found under degrees of illumi- 

 nation similar to those very commonly adopted in observations of 

 tlie sun is amply sufficient to produce by its destruction near the 

 point of contact tlie phenomena which so many observers of 

 experience have declared that they have seen. That the optical 

 enlargement is sufficient for the purpose can be seen from the 

 experiments of Dr. Robinson, of Armagh, and from the Greenwich 

 discussions of eclipse observations. This was pointed out in my 

 letter in your number of August 24. With respect to Mr. 

 Newcomb's remark as to the application of this theory of irra- 

 diation to a transit of a planet, viz., " we require to know 

 whether the irradiation of an extremely minute thread of light 

 darkened so as to be barely visible is the same with that of a 

 large disc, I am decidedly of opinion that it is not, and if not, 

 the fact that the sun's disc is optically enlarged by the telescope 

 or the eye of the observer cannot be directly applied to the 

 phenomena of transit." I have merely to remark that Mr. New- 

 comb is undoubtedly right when he asserts that the irradiation 

 from the minute thread of light darkened so as to be barely 

 visible is not the same as that of the large disc. // is simply be- 

 cause such is the case that the pJienomena of the eonneetiii'^ ii^<'- 

 nietit appear. When the planet is well on the disc, the irradiation 

 around the disc will not be disturbed, but as the planet 

 approaches the edge, the irradiation near the point of contact 

 must eventually be disturbed, and this disturbance, or change, 

 gives rise to the phenomena observed — a black drop, connecting 

 ligament, or whatever name you prefer to give to that apparent 

 cutting out of a piece of the sun's edge near the point of contact 

 which must take place. After the disturbance of the irradiation 

 has once commenced, the connecting ligament must at egress in- 

 crease in breadth ; but I do not profess to be able to give the law 

 of the changing form with any degree of exactness. 



The experiments of Wolf and Andre were, as I stated in my 

 letter of August 24, made upon a disc presenting no sensible 

 tracesofoptic.il enlargement. The results can therefore have no 

 bearing upon the question of irradiation. These results are un- 

 doubtedly valuable in themselves, as showing experimentally the 

 tendency of errors of observations of contacts under feeble illu- 

 mination. They may throw light upon those observations at 

 which no connecting ligament was seen, but they are useless to 

 disprove or prove irradiation effects. 



My authority for stating that the observations of Wolf and 

 Andre were made upon a disc showing no sensible traces of opti- 

 cal enlargement, is contained in the memoir itself. If Mr. New- 

 comb is plea.sed to call the phenomena of " telescopic irradiation " 

 a species of bad definition, there can be no objection on my part 



to his doing so ; but it is not a species of bad definition " which 

 disappears in a fair atmosphere, with a good telescope well ad- 

 justed to focus." 



With respect to the ligament not being a celestial reality. 

 The contact is not a celestial reality. My views of the reality 

 of the phenomena are that the reality is neither more nor less 

 than the reality of the phenomena presented at the focus of 

 an object-glass when turned upon a star. The irradiation can 

 in my view be got rid of to the same extent and in the 

 same manner that the central disc corresponding to the star's 

 image can be got rid of. Vou can reduce its dimensions by 

 cutting down the illumination, and the disc will become a point, 

 but only as it vanishes. 



If I may be allowed to give one word of warning respecting 

 the preparations for the transit of Venus 1S74, it is iinifrri/iity. 

 Make such arrangements as you think best, but once made 

 stick to them even if not absolutely the best. The observations 

 which are to be compared must be made as early as npossible 

 under the same optical conditions. The whole success or failure 

 of the work will, in my opinion, turn upon the extent to which 

 this necessary condition is approximated to. 



E. J. Stone 



Royal Observatory, Cape of Good Hope, Nov. 18, 1S71 



The Origin of Insects 



With your kind permission I will answer Dr. Beale's ques- 

 tions, published in his letter in your issue of December 21, 1S71. 



Dr. Beale asks me what part of the nervous system of the 

 maggot is present in the fly ? My answer is that I have traced 

 the changes of the one directly into the other ; and that Weismann 

 has done the same. There is no time in the pupa state when 

 the nervous system is absent ; but it is difficult to demonstrate 

 this, as amongst so much molecular matter it is not easily found, 

 and is very easily crushed and destroyed. 



Again, Dr. Beale asks me if I have seen any vestige of the 

 mouth organs of the imago in the larva ? I reply that the man- 

 dibles and maxillx exist in the egg twelve hours before the young 

 maggot emerges, together with the fore and hind-head segments ; 

 that these have all disappeared when the egg hatches ; but that 

 the imaginal discs are already formed at that time. Now, I 

 would ask if it bears the slightest aspect of probability that the 

 larval head segments and mandibles, maxillrc, &c., are formed 

 for nothing, and that the imaginal discs are new formations 

 arising contemporaneously with the disappearance of the larval 

 head segments ? Dr. Weismann has shown unmistakably that 

 the abdominal segments of the pupa skin are formed from the 

 abdominal segments of the larval skin. Does it appear in the 

 slightest degree probable that the thoracic and head segments 

 have a totally dissimilar origin ? I admit that I have not been 

 able to see the imaginal discs in contact with the head segments 

 of the embryo ; but I have found the imaginal discs immediately 

 after the egg is hatched, and they are then too much like the 

 embryonic structures alluded to, to have had any other probable 

 origin. The proboscis is formed from cells laid down within 

 these discs ; of that there is not the smallest doubt. Dr. Weis- 

 mann makes the same assertion, and, although I did not know it 

 to be so at the time I wrote my work on the fly, I acknowledge 

 it is so now, and that in my description of the origin of the pro- 

 boscis I was wrong. In the Lepidoptera, and in some beetles, 

 imaginal discs may be seen to have their origin in the inner layer 

 of the larval skin. 



.\gain, Dr. Beale says : — " Does Mr. Lowne mean to say, for 

 instance, that he or anyone else can adduce any reliable observa- 

 tions to prove that the sexual organs are gradually developed, 

 even from the time when the embryo is enclosed within the egg? ' 

 I answer, yes. My own observations confirm those of ^^'eismann 

 on this head, and Dr. Beale will find, on looking again at page 

 112 of my book on the blow-fly, that he has not correctly quoted 

 my statement. I will also refer Dr. Beale to Dr. E. Bessel's 

 paper, " Studien iiber dieEntwicklungder se.xual-Driisen bei den 

 Lepidopteren," in the " Zeitschrifl fiir wissenschaftliche Zoolo- 

 gie," vol. xvii. I believe future observers will find the sexual 

 organs are always so fomied, even as they are in the vertebrata. 

 There is another paper, by Siebold I think, on the same subject 

 in the above quoted periodical. 



Lastly, Dr. Beale asks me to explam what I mean by the sen- 

 tence occurring at page 116 of my book : — "All the tissues of 

 the larva undergo degeneration, and the imaginal tissues are re- 

 developed," &c. I apprehend that the redevelopment of all the 



