424 
NATURE 
[ Sepz. 28, 1871 

phenomena, there must be some corresponding difference in 
the circumstances of observation, and that when among five 
observers three see the phenomena exactly as we know they 
are, while two see them as we know they are not, and even 
then do not agree between themselves, there is a strong pre- 
sumption that the latter do not see them rightly. I am aware 
of but a single attempt to determine experimentally the causes 
why one-third of the observers of the late transit, and many ob- 
servers of former transits, saw the planet distorted, namely, that 
of Wolf and André, to which Mr. Stone alludes. They found, 
in observations of artificial transits under various circumstances, 
that when they used a telescope of at least twenty centimetres 
aperture, with a good object glass, well adjusted to focus, they 
saw only the geometrical phenomena of contact, while, if the 
object glass was small, or not well corrected for aberration, or not 
well adjusted to focus, they saw the phenomena of distortion. * 
In the absence of farther investigation, which is much to 
be desired, these results seem to me, at least, to ‘‘indicate’’ that 
the phenomena in question are due to insufficient optical power, 
or bad definition either in the object glass or the atmosphere. 
At the same time I by no means insist on this proposition as 
established, and it is a great defect in the experiments in ques- 
tion that they do not extend to the effects of using shades of 
different degrees of darknessin observing the sun ; but of this anon. 
In his letter Mr. Stone quotes the observations of Chappe, 
Wales, Dymond, &c., in 1769 ; but I cannot admit that they bear 
strongly on either of the points in question, till we have some 
better evidence than now exists that their object glasses were 
such as Clarke or Foucault would call good. 
Again, the argument from irradiation, if it proves anything, 
proves too much. Ido not see why, upon the theory of Mr. 
Stone, the distortion should not always be seen. To be satis- 
factory, any theory of the matter must explain why it is that 
A, B, C, &c., see the phenomena, while X, Y, Z, &c., do not, 
and that of Wolf and André is the only one which does this. 
Mr. Stone objects to the experiments of the Paris astronomers, 
that their disc was not sufficiently illuminated to exhibit any 
optical enlargement. I do not know his authority for this 
assumption, but, whether well founded or not, it seems to me 
that if the sun were viewed through a dark glass, it would present 
the same optical phenomena of irradiation with a disc so illumi- 
nated as to appear of thesame brilliancy with the darkened sun. 
Thus, in the absence of evidence to the contrary, the Paris expe- 
riments may be taken as showing how the phenomena would 
present themselves in the case of the sun viewed through a shade 
of acertain (unknown) degree of darkness. 
Before we can make any application of the theory of irradiation 
to the phenomena of contact, 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. [ 
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 a 
transit. 
To sum up my views ;—neither Mr. Stowe nor any one else will 
claim that the ligament he saw before the time of internal contact 
was a celestial reaiity—he considers it a result of irradiation, but 
whether of telescopic or purely ocular irradiation I do not under- 
stand. If the former, this is simply a species of bad definition, 
and there is little difference between us. I also admit, on my 
part, that if the telescope and the eye are such that from any 
cause whatever an exceedingly thin thread of light presents itself 
to the sense as a band several times thicker than it reaily is, then, 
as the real thread becomes invisible, the seeming band will appear 
to be broken through by what some may considera ligament and 
others a black drop, and the really sharp cusps will seem to be 
rounded off at their points. If I rightly understand Mr. Stone, 
he holds that these results of the thickening of the thread of 
light by what he considers irradiation are unavoidable. But this 
view is conclusively negatived by the fact that they actually were 
avoided by a large majority of the observers of the late transit. 
Admitting, then, that these spurious phenomena are not unayoid- 
able, it matters little whether we cail their cause irradiation or 
bad definition, though it is important that we should know its 
exact nature. The only attempt I know of to determine this is 
that of Wolf and André, and their results seem to me so nearly 
in accordance with wha we should expect, as to be quite worthy 
of acceptance, at least in the total absence of rebutting evidence. 
Simon Nrwcoms 
* Comptes Rendus, 1868, i. p. 921, 

Solar Parallax 
I HAVE waited ‘somewhat anxiously for Prof. Newcomb’s 
statement of the errors in a chapter on the Sun’s distance 
(‘* The Sun,” Chapter 1.). His review was certainly so worded 
as to imply very gross inaccuracy, and his explanatory letter, in 
which he remarked that more than a column of NATURE would 
be needed for the mere record of my errors, did not improve 
matters. This morning I havereceived his notes. The errors 
enumerated amount but to seven in all; I will leave your 
readers to judge of their importance. 
I. At p. 50, I assign to Hansen’s letter of 1854 the announce- 
ment of the value 8”'9159 for the solar parallax ; whereas this 
value was not announced by Hansen until 1863. TZanguam 
veferat. ansen’s priority remains unaffected by the change. 
2. At p. 53, I mention that Prof. Newcomb deduced a value 
of 8”-84 (probably a misprint for 8” 81) for the solar parallax by 
a certain method. His real result was 8-809. Again my com- 
ment is ¢anguam referat. 
3. At p. 53, Foucault’s ‘‘ parallax is given as 8’’"942, whereas 
the result actually deduced was 8°86,” The matter again is 
utterly insignificant ; but it chances that I have not given Fou- 
cault’s estimate of the parallax as 8942. I remark only that 
if Foucault’s estimate of the velocity of light is correct, the 
parallax would be 8-942. I deduced this result by a calculation 
made on my thumb-nail as I wrote. It is correct, however, and 
Foucault’s was not. 
4. At p. 59, Isay that Mr. Stone deduced the solar parallax from 
observations of Mars made at Greenwich alone, and then 
by combining these observations with others deduced the solar 
parallax at 8943. Now, Prof. Newcomb says that he ‘‘ finds 
no discussion of the observations at Greenwich alone, 7 the paper 
here referred to.” But I refer to no paper whatever. A rough 
calculation of the parallax was certainly made from the Greenwich 
observations alone, though, as Mr. Dunkin remarks at p. 507 of 
his edition of ‘‘ Lardner’s Astronomy,” “the observations by 
this method (single-station observations) were comparatively 
unsuccessful,” ‘* owing to unfavourable weather at Green- 
wich.” Apart from the facts, which fully justify my statement— 
what could the correction be worth in any case? Only the final 
result was insisted upon, 
In a note on this matter, Prof. Newcomb makes “in passing” 
the really important observation that the method of determining 
the sun’s distance by observations on Mars from a single station 
was applied by the Bonds as far back as 1849. Mr, Carrington 
had already told me that he believed the Bonds had anticipated 
the Astronomer Royal. I wrote to Prof. Young asking for 
further information, and was waiting for hisreply, I am obliged 
to Prof. Newcomb foraiding me in this matter. The priority of 
the Bonds inthis matter should certainly be more widely known 
than it is. 
5. Atp. 61. Thisisa very curious correction. I speak of Prof. 
Newcomb as having successfully treated the problem which was 
afterwards discussed by Mr. Stone; and he remarks that 
he knows nothing of the matter, and has read my statement 
with great bewilderment. JI am not responsible for it. There is 
aletter in the Astronomical Register for December 1868, signed 
only **P. S.,” but with unmistakable internal evidence of com- 
ing from the Astronomer Royal for Scotland, in which the follow- 
ing passage occurs :—‘‘I must not say a word about the pyramid 
sun-distance here, or my letter will never be allowed to see the 
light ; something, however, on the score of modern justice to our 
contemporaries | must beg leave to put in. Admirable is the 
praise given to Mr. Stone, and worthy, in so far, the credit 
abundantly bestowed on him at every step of the undertaking ; 
but why is there not one word about Prof. Simon Newcomb, of 
America, having already gone over that same problem similarly, 
and published the results a year sooner?” Of course, as Prof, 
Newcomb now writes that ‘‘he has no recollection of ever having 
made any independent investigation of the observations of the 
transit of Venus,” Prof. Piazzi Smyth was mistaken, and ‘the 
abundant discussions of Prof. Newcomb’s paper in various 
northern scientific societies last winter” (so speaks Smyth) were 
founded on some misconception of its purport. But Prof. Smyth’s 
statements were permitted to remain uncorrected ;—/Ainc ile 
lacryme. 
6. Atpp. 61,62. ‘* The distortion of Venus at the time of in- 
ternal coniact is described as an ever-present phenomenon, and the 
apparent formation of the ligament as conten poraneous with 
true internal contact.” If what I say in pp. 61, 62 admits of being 
so misinterpreted (which I question), the same cannot be said of 
