June 5, 1884 



NA TURE 



an array of facts as a trained geologist can gather in the 

 field, andbyboldly announcing the conclusions to which the 

 study of these facts has led him. But much more may 

 be made of them than he has yet given us. And we trust 

 he may be encouraged to continue the investigation he 

 has so well begun. Arch. Geikie 



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. 

 JVo notice is taken of anonymous communications. 

 [ Th ! 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 Marine Biological Association 



Will you allow me space to ask all naturalists and lovers of 

 science who intend to become members of the above Association 

 to send their names and subscriptions without delay to Mr. Frank 

 Crisp, 6, Old Jewry, London, EX. The subscription is one 

 guinea annually, or fifteen guineas for life membership. It is 

 highly desirable that intending members should at once enrol 

 themselves, since the first meeting of the Association for the 

 election of officers and council for the year 1SS4-85, and for the 

 ratification of by-laws, will be held in London at the cud of this 

 month, when Prof. Huxley will be nominated as President. 

 Donations, whether large or small, are earnestly solicited. 

 Those who are interested in the natural history of marine plants 

 and animals, and who foresee the immense help to this study 

 which a well-equipped laboratory will afford, are begged not 

 only to give some pecuniary aid to the present enterprise, but 

 to constitute themselves agents of the Association and to do 

 their best to persuade others to contribute to the fund required 

 for building the first biological laboratory on the English coast. 

 It is only by hearty and earnest support of this kind that our 

 object can be realised. 



I may add that several naturalists have contributed each 100/. 

 to the Association, others 25/., and others less, according to 

 their means and their sympathy with our object. Of the 10,000/. 

 required, we have not yet obtained half. 



E. Ray Lankester, 

 Secretary {ad interim) 



11, Wellington Mansions, North Bank, N.W. 



The Equatorial Coude of the Paris Observatory 



In continuation of my first letter I now proceed to answer M. 

 Lcewy's second letter, as published in your issue of May 15 

 (P- 52). 



M. Lcewy has not, as I said in that former letter, raised a 

 single objection which had not already been anticipated and dis- 

 cussed with the exception of one which I shall treat of further on. 

 The several points in this letter I shall dispose of very shortly. 



I. As to the dialyte construction, I have to reply that that 

 particular method of achromatising the objective is not an 

 essential feature of this instrument. Whether it be adopted or 

 not is in fact much a question of cost. If the purchaser desires 

 to get the largest possible aperture at least expense, then I would 

 make it a dialyte, for, notwithstanding all M. Lcewy says, 

 good work can be and has been done with dialytes. If, how- 

 ever, the most perfect instrument is desired, I would dispense 

 with the dialyte construction, and achromatise the object-glass 

 in the ordinary way, which is quite as applicable to my con- 

 struction of equatorial as is the dialyte. If I mistake not, the cele- 

 brated observer M. Dembowski observed for many years with 

 a dialyte, and spoke highly of it ; he says : " Eachromatisme est 

 excellent." Again, the present director of the observatory, for 

 whom the first of these instruments is to be made, has worked 

 already with dialytes, and he would not be likely to recommend 

 this construction if his experience agreed with M. Lcewy's. I desire 

 to notice just one further point in this part of M. Lcewy's letter, 

 as it is another example of how his own words (unintentionally, 

 no doubt) confirm my statements. He says (speaking of the 

 limited field of view of dialytes) : "But, in order to turn the 

 difficulty, he" (Mr. G.) "suggests that since the field of view 



becomes smaller as the instrument becomes larger, we may 

 content ourselves villi observing at a central point." I never 

 said this : my words were: "The definition at the edge of the 

 field, however, is not so good as in the ordinary form, I ml this 

 would not be of so much consequence in large instruments, as 

 the field in such cases is never of great extent.'' And M. ! cewy 

 himsell corroborates this for me when he says : " For 1 

 vation oi comets 1 have such an eyepiece, which magnifies fifty 

 limes, and has a field of view such that I can observe a di igree 

 {i.e. with the 12" equatorial coude) ; for a telescope of 27 inches 

 we might have such an eyepiece with a field of 24 minutes." 

 Thus I have a distinct corroboration from M. Lcewy of what I said 

 above. 



2. Writing on the matter of stability, M. Lcewy curiously 

 mixes up stability and accuracy of movement. Now while I 

 claim that I can and will obtain greater stability in my form 

 than exists in M. Lcewy's, I do not claim accuracy of movement, 

 but on this point I propose to say very little at present for several 

 reasons. In the first place, it would hardly be possible to dis- 

 cuss this and put it in an intelligible form to your readers with mt 

 a careful drawing ; secondly, the well-known stability of the 



nl > which have emanated from my workshops are quite 

 sufficient guarantee that this point is not one likely to be 

 neglected in any of my work ; and thirdly, I find it utterly im- 

 possible to understand the sentences of M. Lcewy's paper bear 

 ing on this point, and if I, though familiar with the proposed 

 construction, fail to understand them, I am hopeless of serving any 

 useful purpose by discussing them in your columns, particularly as 

 few of your readers have ever seen the design of the instrument 

 referred to. M. Lcewy talks of "all movements of transmission 

 being broken at right angles." I do not know what he means, 

 but he omits to tell your readers that, according to my design, in 

 the larger sizes I propose that all movements be effected by two 

 hydraulic cylinders the valves of which are within reach of the ob- 

 server while sitting in his chair ; so that, without more pin 1 

 exertion than is necessary to open a water-tap, he has full com- 

 mand of all the movements of the great instrument, a pair of 

 vertical scales on the walls of his study giving the approximate 

 position of instrument in /R and declination, an arrangement 

 eminently calculated to reduce the work of the observer. 



3. Lastly, as to its want of universality. This is distinctly 

 stated in my paper as a disadvantage of my form ; but when M. 

 Lcewy asserts that " it is based on a principle which no astm- 

 nomer can admit, viz. that it is superfluous to observe the greater 

 part of the northern heavens," it is evident that M. Lcewy has 

 here gone too far, since that portion of the heavens within 20 

 of the Pole is only about 6 per cent, of the northern hemi- 

 sphere. Ask any practical astronomer possessing a moderate- 

 sized equatorial how many hours out of the total number of 

 hours which he has worked in the year has his instrument been 

 pointed to objects within 20° of the Pole, and, with the exception of 

 a few who apply themselves to special work, the great majority 

 will give a reply which will show how very little will be lost by 

 the. fact that this instrument cannot command that portion of the 

 heavens. I have myself put this question to many, and with 

 the result above mentioned. On this point I cannot do better 

 perhaps than give an extract from a letter I have just received 

 from the director of one of our public observatories : — " Instru- 

 ments of large aperture are rarely if ever used for observations 

 where extreme accuracy of measurements is required, such as 

 annual parallax, nor for searching for nor observing comets, 

 except to search along a known track for an expected periodical 

 comet. This your instrument could do well. There is hardly 

 an instrument in existence which is equally well adapted to all 

 kinds of observations. The circumpolar zone of about 20" 

 may lie explored by other instruments, but for almost every 

 kind of systematic work the remainder of the visible heavens will 

 give plenty to do." The foregoing would be a sufficient answer 

 to a question which M. Lcewy has put directly to me. 



He says : — " Permit me to ask Mr. Grubb how he is goine; to 

 study that part of the heavens which lies between 20 from the 

 zenith and the Pole." 



To any one who has seen my paper it will be evident that 

 this point, which in M. Lcewy's letter is put forward as a dis- 

 covery of his own, was already fully dealt with by me. I said : 

 " The instrument commands the heavens from east to west and 

 from south horizon to about 20° beyond zenith." And again: 

 "As regards this instrument (equatorial coute) I would observe 

 that it possibly possesses an advantage over my form in being 

 absolutely universal." 



