NA TURE 



[A/ay 29, 1884 



of Eqtiisetum are composed of cells. As is usual in works 

 such as this, the terminology of the reproductive organs of 

 the lower forms is very erratic : thus he uses the terms 

 " spore " and " antheridium " as equivalent in Selaginella 

 (P- 1 39), he calls the "sporogonia" of the Mosses 

 " sporangia,' - and the " oogonia " of Fi/a/s " perispores." 



On the subject of the lichen-gonidia theory he waxes 

 warm, stating (p. 150) that it has been "termed sensa- 

 tional romance by every well-known practical fungologist 

 and lichenologist." Of course every one is free to express 

 his own opinion, but few who are not blind partisans will 

 be prepared to agree with Mr. Step in excluding such 

 men as De Bary, Schwendener, and Stahl from the list of 

 " well-known practical fungologists and algologists." 



Till so-called popular books are written with more accu- 

 racy, we should strongly advise those who wish to dabble 

 in science either to abstain, or, better, to brace their 

 minds to attack some text-book which can be depended 

 upon : after this, if they wish, they can easily supply for 

 themselves that cheap sentiment with which " Plant-Life " 

 abounds. 



LETTERS TO THE EDITOR 

 \ The Editor does not hold himself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to return, 

 or to correspond with the writers of, rejected manuscripts. 

 No notice is taken of anonymous communications . 

 [The Editor urgently requests correspondents to keep their letters 

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

 that it is impossible otherwise to insure the appearance even 

 of communications containing interesting and novel facts .] 



The Equatorial Coude of the Paris Observatory 



It was only on the 7th of this month that my attention was 

 called to a letter in Nature (p. 4) from M. Lcewy, headed 

 " Reply to Mr. Grubb's Criticisms on the Equatorial Coude of 

 the Paris Observatory." I deferred answering this letter as I 

 was travelling at the time, and had not the necessary data by me 

 to refer to ; further, because I desired to see the second part of 

 M. Loewy's letter before making my reply ; and lastly, because 

 I hoped that some of the errors in M. Lcewy's first letter were 

 misprints, and that he would have corrected them in his second. 



I may observe in passing that the paper to which M. Lcewy 

 alludes is a description of an instrument in the construction of 

 which 1 am at present engaged, and that any criticisms of the 

 equatorial coude which it contained were merely incidental. 



M. Lcewy assumes that my instrument is a modification of one 

 which he described in 1871 in the Comptes Rendus. This de- 

 scription I have, as it happens, never seen, nor am I much con- 

 cerned to defend myself from the suggestion of plagiarism, since 

 the fundamental principle of what I have called the "sidero- 

 static telescope " is one the advantages of which were recog- 

 nised long before 1S71, although mechanical difficulties prevented 

 its application. 



I find some difficulty in rendering my reply intelligible to your 

 readers, for although the equatorial coude has been described 

 and figured in NATURE, they are not ns yet familiar with the 

 form of the instrument which I am at present constructing. I 

 now proceed to consider M. Lcewy's first letter. 



I cannot admit that M. Lcewy's recital of what he is pleased 

 to call my criticisms is accurate. He says : — 



" To give weight to his argument Mr. Grubb examines a case 

 of the construction of an instrument of 27 inches aperture, and 

 he anticipates in the construction the following difficulties which 

 he considers insurmountable : — (1) The optical difficulty of con- 

 structing a large plane mirror. (2) The practical difficulty of 

 procuring a disk of the necessary dimensions. Mr. Grubb 

 affirms there is no glass-works capable of making a disk of glass 

 so large. (3) The difficulty of moving a mirror of which the 

 weight, according to Mr. Grubb's calculations, will be very nearly 

 half a ton. (4) The dearness of the instrument, which would cost 

 more than an ordinary equatorial plus dome and observatory." 



With regard to (1) I may remind M. Lo?wy that I distinctly 

 admitted the "possibility" of constructing such a mirror, but I 

 expressed the opinion that inasmuch as no mirror of this size had 

 yet been attempted success was problematical. 



I now learn from M. Lcewy that such a mirror has been made, 



and that it "leaves absolutely nothing to be desired" ; but he 

 has not told us to what tests this mirror has been subjected. I 

 am not aware that there exists in Paris an object-glass of suf- 

 ficient size to embrace the whole pencil of light reflected from 

 it, and without this it is not possible, in my opinion, to apply 

 tests which can be legitimately considered final. 



(2) I did say that I believed it- to be impossible to obtain disks 

 of such large dimensions, and I founded my belief on the fact 

 that not many years ago I applied to several of the principal 

 glass manufacturers of England, France, and Germany for prices 

 of disks for mirrors, the weight of which would have been less 

 than those mentioned by M. Lcewy, and none were willing to 

 undertake the work. I am glad, however, to learn from M. 

 Lcewy that the glass manufacturers are now prepared to furnish 

 disks of such a size. 



(3) The next difficulty M. Lcewy attributes to me is "the 

 difficulty of moving a mirror of which the weight, according to 

 Mr. Grubb's calculations, will be very nearly half a ton, " and 

 this, he says, " has arisen from some error in calculation." 



I would ask M. Lcewy to point out where I said anything of 

 the difficulty of moving a mirror of half a ton weight. As I had 

 a considerable share in the construction of the Melbourne mirrors, 

 which weighed nearly four times this, it is hardly likely that I 

 would assert such an absurdity ; and as to the allegation of a 

 wrong calculation, I ask M. Lcewy to point out the mistake he 

 refers to, and in order to give him every facility for so doing I 

 append it below. It is necessary first, however, to settle the 

 matter of thickness. 



M. Lcewy, in Nature, May 1 (p. 5), says : — " I have never 

 said that the thickness should be_p - l8 of the diameter. I have 

 given O'lS as a maximum." 



I have, however, before me a paper presented by M. Lcewy to 

 the Academy of Sciences, March 19, 1S83, in which occurs the 

 following passage: — " Mes recherches m'ont demontre que 

 pour prevenir dans tin miroir toute deformation causee par la 

 flexion ou un leger serrage, il faut que l'e'paisseur du verre soit de 

 O'lS du diametre." Now if I understand this rightly, it means 

 that he has found o - l8 of the diameter to be the minimum 

 thickness necessary, and in no part of that paper does he speak 

 of o - l8 as a maximum. He mentions in the following para- 

 graph that a thickness of one-fourth is necessary under certain 

 conditions ; and further on in the same paper he says : — " C'est 

 dans ces conditions que les miroirs du nouvel instrument ont ete 

 construits." This being so, I still think I was justified in 

 assuming 0^225, a mean between one-fifth and one-fourth, as that 

 which he recommended. Now as to diameter. He assumes a 

 diameter of 38 inches to be sufficient for a 27-inch objective, but 

 27 X V2 = 38-178. Even this would only give full aperture of 

 objective for the central pencil. For a moderate field nearly 

 3S£ would be in actual use. 



Again, no optician in practice thinks of making a mirror with- 

 out some margin ; in small sizes one-fourth to one-half of an 

 inch, but in such a size as this three-fourths of an inch all round 

 is not an extravagant allowance, and therefore in adopting 40 

 inches diameter I did not exaggerate. 



I therefore had as my data — 



Diameter 40 inches. 



Thickness 40 X 0'225 = 9 inches. 



The calculation then is simple. 

 „ ,., . . ,. , . Area of 40 inch circle X 9 



Solid contents in cubic feet = 



1728 

 — 6 '547 cubic feet ; 

 and, taking a specific gravity of 2 - 5, each cubic foot would weigh 

 I56jlbs. ; therefore the weight of mirror = 6-547 X I56'25 = 

 1023 lbs., or rather mere than 9 not. I said nearly half a ton. 



I may add that the specific gravity of the glass supplied to me 

 by Messrs. Chance is rather more, viz. 2-52, which would give 

 a higher result. 



In another place M. Lcewy represents me as finding that 

 "the mirror necessary for an equatorial coude of 27 inches 

 would weigh 8i tons " ! ! 



I never made such a statement, and I challenge M. Lcewy to 

 say wdiat grounds he has for this assertion. 



This is the second case in which he attributes to me statements 

 which I have not made. A third and a fourth we shall come to 

 just now. Your readers will remark that M. Lcewy (except in 

 one minor instance) never quotes my words. Hence perhaps 

 have arisen the serious mistakes into which he has fallen. Re- 

 specting the matter of thickness, M. Lcewy appears to have 



