Nov. 2, 1 87 1 J 



NA TURE 



7 



hand simultaneously with the image of the object-glass or specu- 

 lum formed by the eye-piece ; the diameter of that image is given 

 at once by the divisions to j-J-j of an inch, and can be readily 

 estimated to half that value. The arrangement mentioned by 

 " W. R." is no doubt very convenient, and quite adequate for his 

 purpose ; but for high powers I should suppose that the com- 

 parative coarseness of the engraved lines would make itself much 

 more felt than it is in Mr. Berthon's invention, and the balance 

 of economy is so greatly in favour of the latter in comparison 

 with every contrivance with which I am acquainted, that I have 

 no hesitation in saying that it ought to bo in the hands of every 

 amateur who cares to know the magnifying power of his tele- 

 scope. It may be procured for five shilUngs, of Mr. Tuck, 

 •watchmaker, Romsey. T. W. Wei!B 



New Form of Cloud 

 The kind of cloud described by M. Andre Poiiy (Nature, 

 Oct. 19, 1S71, p. 489) is by no means new or rare if I can 

 judge correctly from the figure and explanation. It may often 

 be seen on the lower part of the flank of a great rain or thunder 

 cloud, and appears to arise from the dropping or subsidence of 

 portions of the air heavily loaded with watery particles. My 

 own impression is that it appears when the cloud is about 

 to break up. M. Poey will find the cloud described in the 

 Philosopliical Magazine for July 1857, where the nzxazoi droplets 

 is given to the form, and its position in a thunder cloud indicated 

 by a figure. J- 



Spectrum of Blood 



In my letter, published in the last number of Nature, I 

 am strangely enough made to say that "we must not rely on 

 /Z;!" spectrum." This is an error of the printer. The sentence 

 should have been : — " I have always argued that in such inquiries 

 we must not rely on one spectrum, but compare the action of 

 various reagents." H. C. SoRBY 



Broomfield, Sheffield, Oct. 28 



Earthquake in Burmah 



I HAVE not read in your "Notes" any record of the earth- 

 quake which was felt at daylight of the i6th February last in 

 this city, in two successive and gentle but decided shocks, doing 

 no damage, but which, from the files of the Calcutta Eng/is/iman 

 of February 18, seems to have been severe to the N. W. of this, 

 extending through Cachar, Silchar, Gowahatty, to Calcutta and 

 Barrackpore. 



This earthquake, you will observe, is synchronous with those 

 of the western hemisphere already recorded by you. 



Charles IIalsted 



Mandalay, Burmah, Sept. I 



A Plane's Aspect 



I AGREE with Mr. Proctor that the disuse of the term 

 "position" in geometry would be a serious misfortune ; happily, 

 however, it is not its disuse, but the prevention of its misuse 

 which is contemplated. I cannot agree with him that " position " 

 is a word "which no one can misunderstand," for his own letter 

 is a striking example of its being misunderstood, either by Mr. 

 Proctor, or by others. " Aspect and slope," he tells us, " indi- 

 cate two elements, which, together, fix the position " of a plane. 

 Geometers, however, certainly understand, when a plane is said 

 to be given in position, that something more than its aspect and 

 slope may be regarded as known. Parallel planes have neces- 

 sarily the same slope and aspect, but surely not the same position. 

 To be told that, because its slope and aspect are invariable, the 

 plane of Saturn's rings has a fixed position in space, notwith- 

 standing that the planet moves bodily in its orbit, would 

 scarcely satisfy a student of astronomy accustomed to geometrical 

 precision. 



There can be no doubt that "position" is the true English 

 equivalent of the German word " Lage," and that no ambiguity 

 of the kind above indicated could attach itself to the term, had 

 we asuitable English rendering for the word " Stellung." I do 

 not consider the term "aspect "to be perfect as an equivalent 

 for "Stellung," but I have no hesication in admitting that Mr. 

 Laughton's suggestion is happier than any previous one I can 

 remember. Mr. Proctor declares his intention of opposing the 



' ' use of the word ' aspect ' in a sense not at present assigned nor 

 properly assignable ; " but when he wrote thus, he had not seen 

 the letter of Mr. Wilson wherein the term "aspect " is very rigidly 

 defined to be the direction of the normal. To me this very 

 facility with which the word "aspect" lends itself to rigid defini- 

 tion, is a grounl of objection against it. I have never seen 

 Stellung defined in the manner in which Mr. Wilson has defined 

 "aspect." Von Staudt, in whose admirable writings I first met 

 the word, introduced it thus : " Parallel planes possess some- 

 thing in common, which may be regarded as appertaining to 

 each one of them, and shall be called their ' Stellung ; ' the 

 ' Stellung ' of a plane, therefore, is determined by any plane 

 which is parallel thereto, and two planes have the same ' Stel- 

 lung ' or different ' Stellungen ' according as they are parallel to, 

 or intersect one another." 



That the term " aspect " is not sufficiently elastic to permit of its 

 taking the place of " Stellung " in the above passage cannot, I 

 think, be well maintained by Mr. Proctor, seeing that he has 

 not himself hesitated to use it in two widely different senses in 

 the following passage of his letter : " I can see no reason why 

 'aspect' should be regarded in a new and unfamiliar aspect." The 

 expression "aspect of a plane," whether it be retained or not as 

 the equivalent of the "Stellung einer Ebene," appears to me, I 

 confess, to be much too good to be claimed by Mr. Proctor as 

 indicative solely of the direction of the projection of t/ie normal 

 upon a certain plane of reference. I would suggest, in the interest 

 of his twelve excellent books, that he might qualify "aspect," 

 as thus defined, by an appropriate adjective, for the term is there 

 used in a very technical sense indeed, and is not even applicable 

 to all planes. Although Mr. Proctor can assign, for example, a 

 southerly aspect to the tace of a roof which has a slope ol 30°, 

 he would find some difficulty in describing the aspect of a roof 

 which has no slope at all, whereas Mr. Wilson would without 

 hesitation pronounce its aspect to be vertical. 



Athenreura Club, Oct. 31 T. Archer Hirst 



It is due to my friend and your correspondent, Mr. Cecil J. 

 Monro, of Hadley, to state that, to my knowledge, he was in 

 the habit of employing the word " aspect " in this technical 

 sense long before the publication of Mr. Laughton's letter, and I 

 should not be surprised to learn that other geometers have used 

 it before Mr. Monro. 



I think Mr. Proctor will find few to agree with him in his con- 

 demnation of the word so used. For myself I heartily agree 

 with Mr. Wilson in the welcome he accords to this " old friend 

 with a new face." C M. Ingleby 



Highgate, N., Oct. 27 



I AM glad to find, by Mr. Wilson's letter in Nature for 

 October 26, that the word "aspect," which I suggested, is 

 accepted by him as satisfactory ; as, in fact, the woid wanted. 

 But another correspondent in the same number, Mr. Proctor, 

 pertinaciously insists on the superior merit of the word " posi- 

 tion," to be used in the particular sense explained by Mr. Wilson 

 in his former letter. In this I conceive Mr. Proctor is entirely 

 wrong. 



" In geometrical language " — I quote from Gregory's " Solid 

 Geometry," 1845 — "the position of a plane is determined by 

 making it pass through three given points." Mr. Proctor says 

 he "can see no reason why ' position' should be dismissed from 

 the position it has so long occupied. " No more can I. I would 

 only call his attention to the fact that the meaning whicli he 

 would assign to the word "position " is quite different from that 

 which has been accepted, in a technical sense, by geometers, 

 and in an everyday sense by everyday people. 



Mr. Proctor's special objections to the word " aspect," rest, it 

 seems to me, on a misconception of its meaning and familiar use. 

 We speak of the aspect of a wall, but not of the aspect of a roof, 

 nor of a hill. What the usage amongst builders in respect of 

 roofs may be, I don't know, but geographers almost invariably 

 speak of the "slope" of a hill, as, for instance, the southern slope 

 of the Himalayas. Put into exact language, the as]3ect of a 

 plane is the direction of its normal ; and as par.allel planes have 

 parallel normals, any number of parallel planes have the same 

 aspect, without reference to their position ; but no two planes, 

 parallel or not, can possibly have the same position. 



The word "slope" is almost equally inadmissible; in ihe 

 first place, it refers to some other plane, which is apt to cause 



