I 82 



NATURE 



{Jan. 4, 1872 



" The sun's rays are the ultimate source of almost every 

 motion which takes place upon the surface of the earth. By 

 its heat are produced all winds and those disturbances in tlie 

 electric equililjrium of the atmo'^phere, which give rise to the 

 phenomena of terrestrial magnetism. By their vivifying action 

 vegetables are elaborated from inorganic matter, and be- 

 come in their turn the support of animals and men, and 

 the sources of those great deposits of dynamical efficiency 

 which are laid up for human use in our coal strata. By them 

 the waters of the sea are made to circulate in vapour througli 

 the air and irrigate the land, ])roJucing springs and rivers. By 

 them are produced all disturbances of the chemical equilibrium 

 of Nature, which by a series of compositions and decompositions 

 give rise to new products and originate a transfer of materials. " 



In a note in Mr. Herbert Spencer's "First Principles " (2nd 

 Ed., p. 496), which first led me to look at this passage, it is re- 

 marked that Herschel "expressly includes all geologic, meteoro- 

 logic, and vital action*, as also those which we produce by the 

 combustion of coal," in the effects of the solar rays. When, 

 therefore, Prof Tyndall states that Mayer i\'vnli-d the source 

 of the energies of the vegetable world, it appears to me 

 that Herschel anticipated the revelation twelve years previously. 

 Of course I apprehend that Mayer's merit consisted in seizing at 

 once a physical principle of immense generality, and in applying 

 it to very different phenomena. Herschel began at the other 

 end ; but appears equally to have seen the solar energy under- 

 lying these phenomena, though in a general way, and without 

 demonstrating numerical relations. 



2 De Saussure is credited unreservedly with the observation 

 of the reducing power of the solar rays in the vegetable economy. 

 But he seems to me, as, indeed, he seemed to himself, to have 

 only crowned a theory which other workers had elaborated. 

 Priestley began by ascertaining that air dejiurated by animals 

 was purified by plants. Ingenhousz showed, what Priestley can- 

 didly confesses he missed, that this effect is due *' chiefly, if not 

 only, to the light " of the sun. Senebier found that " fixed air " 

 was the ingredient which plants removed from a vitiated at- 

 mosphere, and that this underwent elaboration in the tissues, 

 oxygen being set free as the result.* Lavoisier having 

 previously shown that fixed air was a compound of carbon 

 and oxygen, Senebier's results implied the fixation of carbon by 

 plants. This fixation De Saussure actually demonstrated by 

 feeding a plant with carbon dioxide and water alone, and show- 

 ing that the carbon in the tissues increased. He further found 

 the unexpected fact (and this is what he .added to the matter) 

 that the oxygen evolved by plants does not correspond to that 

 contained in the carbon dioxide absorbed, but that it is smaller 

 in quantity. 



lie .Saussure's researches are a beautiful example of quantita- 

 tive work, but they would have, I imagine, merit of a different 

 order if Priestley, Ingenhousz, Senebier, and Lavoisier had not 

 broken ground before them. 



W. T. Thiselton Dyer 



Phenomena of Contact 



In Nature of August 24 I objected to, as misleading, the 

 statement by Mr. Newcomb that "\\t find ligaments, black 

 drops, and distortions sometimes seen in interior contacts of the 

 limbs of Mercury or Venus with that of the sun, described as if 

 they were regular phenomena of a transit, without any mention 

 of the facts and e.xperiments which indicated that these pheno- 

 mena are simple products of insufficient optical power and bad 

 definition which disappear in a fair a'mosphere with a good 

 telescope well adjusted to focus." I asked for references to the 

 facts and experiments by which the statements are justified. 



In Nature of September 2S I find Mr. Newcomb's reply, but 

 without the references which I desired. Mr. Newcomb considers 

 that I controvert the two following propositions : — 



1. That the irregular phenomena of internal contact of a 

 planet with the sun, variously described as distortions, black 

 drops, ligaments, &c., are not always present, but are only seen 

 sometimes. 



2. That when seen they are due to insufficient optical power 

 or bad definition. 



If the word "irregular " is cut out, and the word "seen " sub- 

 stituted for "present" in proposition (i) there can be no doubt 

 about its truth. It will be found that all the arguments adduced 



* Recherches sur I'influence de la lumifere solaire pour nietamorphoser 

 I'air fixe en air pur par la v6g6tation, 1783. 



by Mr. Newcomb to prove this proposition have no bearing 

 either upon the word "irregular " or " present " in contradistinc- 

 tion to " seen." 



It appears to me, therefore, quite unnecessary to allude 

 further to ihis proposition. . 



With reference to proposition (2), I believe it to be utterly 

 erroneous. I believe that the phenomena of the fine connecting 

 ligament can only be seen in a fair atmosphere, with a good 

 telescope well adjusted to focus, and with considerable magnifying 

 power. When it is remembered that the fine connecting liga- 

 ment is confined to within about a second of arc of the sun's 

 limb, I think my statement will at least commend itself to prac- 

 tical observers. Mr. Newcomb appears to regard it as a great 

 difficulty in my view of these phenomena, that some of the 

 observers should see the ligament and some not. I am rather 

 surprised at the persistence witli which this point is again and 

 again brought forward in his letter. I thought that it had been 

 answered by anticipation in my letter which appeared in your 

 number of August 24. In all my writings upon the subject I 

 have maintained that the phenomena could only be seen under 

 favourable circumstances and with sufficient power ; and in my 

 letter of August 24 will be found this statement, which appears 

 to have been entirely overlooked, at lease unanswered, by Mr. 

 Newcomb : — " The optical enlargement by irradiation is a Unc- 

 tion of the brightness, and can be made insensible by sufficiently 

 diminishing that brightness. Unfortunately, however, when this 

 diminution of brightness is carried to a very great extent, errors 

 in an exactly opposite direction to those of irradiation will come 

 into pUy, similar, in fact, to the results of Wolf's experiments. 

 The observations of Mercury on the sun's disc in 186S were 

 nude with very different optical means, and some very different 

 methods were adopted for diminishing the sun's glare." In my 

 view those observers who did not see the conneciing ligaments 

 failed to see it, either from want of attention to the point as 

 not a contact such as they expected to see, or from the observa- 

 tions having been made under such circumstances that some of 

 the necessary conditions which I have indicated were not satisfied. 



The fine connecting ligament is only seen by contrast against 

 the illumination of the sun's disc near the point of contact, 

 and it may well be that some of the observers have pushed the 

 diminution of brightness of the sun's image to such an extent that 

 the contrast was too feeble to attract attention before the appa- 

 rent contact. To me, and I think to 01 hers who will give the 

 matter some consideration, it is clear enough " that an observer 

 with the naked eye, a telescope of low power, would not, in the 

 case of a transit o£ Venus, see the connecting ligament at all." 

 It is as clear "that without seeing any ligament, the planet, at 

 egress, would appear to touch the limb without distortion, neces- 

 sarily, earlier than the contact would appear to be established to 

 observers who were watching the transit with good telescopes and 

 with high powers." It appears to me equally clear that, if the 

 brightness of the sun's image be reduced to excess, then the ever- 

 diminishing small portion of the illuminated disc between the 

 sun's edge and the advancing planet, at egress, may be made to 

 disappear, from sheer inability to appreciate so faint a light, before 

 the contact would appear to be established to observers who had 

 not so reduced the brightness of the image. Disturbing causes 

 such as these do exist, and their effects must be recognised. I 

 must apologise for bringing forward such arguments ; but, since 

 one obseiver has published his opinion that the "ligaments, 

 c&c," do not exist in contacts, because he looked at the transit 

 of Mercury with an opera glass and saw nothing of the kind, it 

 would appear necessary to recall attention to a common-sense 

 view of the points at issue. 



But whatever may be the opinion of Mr. Newcomb respecting 

 the explanation which I have given of the probable reason why 

 some of the observers have not seen the connecting ligament, he 

 must feel that it will at least be difficult fiir him to explain away 

 the positive evidence of the numerous observers who profess to 

 have seen the ligament with first-rate telescopes. Some of them, 

 at least, were gentlemen not likely to have forgotten to adjust 

 their eye-pieces to focus, even if such a neglect would have pro- 

 duced the phenomena observed. In cases where the ligament 

 has been seen, it will be found that the earlier lines of contact 

 at egress have been given by (ibservers with the best telescupes 

 and high powers. This is strikingly shown by the Greenwich 

 observations of the transit of Mercury, 1S68. It is a result in 

 perfect accord with my views. Very large and systematic dis- 

 cordances will be found to exist between the times of internal 

 contact at the transit ot Mercury, i858, in cases where no con- 

 necting ligament was seen at all. This has been passed over in 



