492 



NATURE 



\Oct. 17, 187: 



should have written occasionally with looseness and 

 inaccuracy ; for example, at p. 52 we find "Vertical iron 

 at the same place will produce the same deviation in 

 whatever direction the ship's head may be, because a ver- 

 tical line makes always the same angle at the same place 

 with the line of force." There is confusion of cause and 

 effect here — the deviation is not the same in whatever 

 direction the ship's head may be, although the force re- 

 mains constant or nearly so— and these are important 

 features for the student to realise. 



Notwithstanding, however, these shortcomings. Dr. 

 Merri field's " small manual " may in the main be received 

 as an orthodox pi'oduction, and will tend to divert the 

 attention of his students from some singular statements 

 and conceptions relating to the deviation of the compass 

 on board ship and its compensation, to be found at pp. 

 20-23 of his Navigation and Nautical Astronomy published 

 conjointly with H. Evers in 1S6S. 



We hope to see manuals of this smaller class ema- 

 nating from seamen, who, possessed of the knowledge of 

 the exact requirements in the practice of their calling, 

 can extract from original sources, and simplify the main 

 points in the various divisions of the subject. 



LETTERS TO THE EDITOR 



[ The Editor does not hold himself responsible for opinions expressed 

 by his correspondents. No notice is taken of anonymous 

 communications. ] 



Solar Spectroscopic Observations 



I HAVE just seen Herschel's letter lo you in Nature of 

 October 3, and am induced to address you, lest his remarks on 

 Indian Climate should make difficulties in respect lo tlie British 

 Association's proposal for a physical observatory for solar 

 observation. 



I do not think Captain Ilerschel has been in England since 

 the Eclipse of 1S68, and consequently he cannot speak from ex- 

 perience of the English climate. It is beyond all doubt that 

 from some cause (commonly believed to be haze) there are really 

 veiy few days when the protuberances can be studied m England. 

 Herschel's letter contains evidence that that cause, whatever 

 it b;, is absent in India. We need not go further ; we have the 

 two facts { I ) that it is impossible in England, with great tele- 

 scopic power and great dispersion, to see the prominence lines 

 regularly ; (2) that it is possible to do so in India, even in the 

 plains at a bad season. The result seems to be that solar inves- 

 tigations should be pursued in India systematically. 



I may add that the facility of seeing these things, and the pro- 

 bability that spectroscopic in'piiries into the celestial bodies would 

 have been more successful in the earlier stages had there then 

 been any observei^s in India, are to my mind a strong argument 

 for now establishing in Indi.i, and on a permanent footing, an obser- 

 vatory, whose speciality shall be " Researches on the Physics of 

 the Sun and Planets," whether by the spectroscope, or vision, or 

 photography. When the proposal of the British Association is 

 made formally to the Government by the Council, it will, I 

 trust, take this form. There is no such institution now in the 

 British dominions, and when one is established, it should be 

 in the Tropics somewhere, and as systematically devoted to 

 physical researches as Greenwich has been to the moon. 



The Director might, probably would, as at Greenwich, in time 

 attach other investigations ; but these researches should be his 

 primal object ; and, if they were made so, there is no reason why 

 work as standard and as useful should not result . 



October 4 J. F. Tennant 



Consciousness and Volition 

 The question raised by Mr. Bennett in last week's Nature 

 is of great importance, and of no small difficulty. During a 

 visit to die late Sir WiUiam IlamUton in 1S55, this subject came 

 up for discussion. Sir William was then engaged on his edition 

 of the works of Dugald Stewart. I called his special attention 

 to Stewart's doctrine regarding the operation of Will in acts 

 which are usually ascribed to Habit. Stewart asserts that all 

 habitual actions are really voluntary. As he had no acquaintance 

 with the modem doctrine of "latent mental modifications," he 



would naturally take for granted that there can be neither a voli- 

 tion nor any other act of mind without accompanying conscious- 

 ness. He accounted for the non-remembrance of that conscious- 

 ness by the cxtre;ne rapidity of the volitional action. (Collected 

 Works, vol. ii. chap, ii.) To this Sir William objected. lie 

 went on to show tl.at in many cases an act, or even a long series 

 of acts, originally voluntary, have ceased to be so. The habit or 

 habitude, which is a mental tendency, though not a pow.r, 

 generated by cust im, supplies the place of volition. In illustra- 

 tion, he referred ]i.rrticularly to the well-know-n fact that in India 

 soldiers will march long distances when they are asleep. Now, 

 it seems to me that this decides Mr. Bennett's question. Here 

 we have regulated action, determined, not by volition, but by 

 habit. Sir William, however, failed to meet all my difficulties, 

 because, as I afterwards saw, of his unsatisfactory theory of 

 caus^tion. He so frequently confounded conditions with causes. 

 With him a cause denoted anything without Which an effect could 

 not be ; hence his doctrine of con-causes, a plurality of causes 

 for each effect. Thus, when I will to move my hand, and the 

 movement follows, Sir William would call the volition one 

 cause of the movement, whereas it is merely a condition. It is 

 remarkable that he should fall into this error since he rejected 

 ISiran's doctrine regarding the efficiency of volition. In a sub- 

 sequent conversation we discussed the points of similarity and 

 the points of difference between Habits and Instincts. It would, 

 however, be trespassing on your space to give the details. 



I will only add that an " unconscious volition " in the sense 

 intended by Mr. Bennett is not possible. Dr. Carpenter's ex- 

 presion "unconscious cerebration" I regard as unfortunate, 

 since it appears to rest on the assumption of the essential identity 

 of mental, vital, and physical powers. Mr. Bennett will find 

 many valuable observations on the nature and conditions of 

 habitual acts in Hamilton's lectures on "Metaphysics," Morell's 

 " Outlines of Mental Philosophy" (published in 1862), and Sir 

 Henry Holland's ' ' Mental Physiology. " 



John Moore 



Stamford House, Sale, near Manchester, Oct. 14 



The Solar Spectrum 



Upon reading the communication from Capt. Ilerschel in 

 your number for October 3, upon the solar spectrum, I seem to 

 remember a letter from a correspondent being published some 

 year or more ago in your pages, in which it was stated the 

 writer had seen the bright lines near the sun's liinb with one of 

 Browning's direct vision prism spectroscopes, the instrument 

 being placed on the back of a swing looking-glass as a stand. 

 Tile dispersion of this instrument would be probably rather more 

 than that of one angular prism. I am bound to say that I have 

 been unable myself, up to the present time, to do more than see 

 a bright line near D, superposed on the solar spectrum, with 

 such an instrument as, however, for other purposes, is a most 

 convenient and sufficiently powerful one. 



To my own I have added three small wedge-shaped pointers cut 

 out of thin brass, and fixed in the eye-piece, while the slit-plate, 

 and consequently the spectrum itself, is drawn across the field by 

 a micrometer screw having a range of about 600 divisions between 

 its starting point near A and G at the other end of the spectrum. 

 In this way a bright line (say the auroral one) may be brought 

 upon one of the pointers, and its distance from D, in a salted 

 spirit-lamp flame, at once accurately measured and mapped 

 down ; and if the pointer is again brought on the line after the 

 observation, it may be verified by the position of the pointer 

 next day upon the solar spectrum itself 



It occurred to me the other evening to try the effect of a 

 V-shaped slit, and it seems to me to have some advantages. 

 The lower ends of Inight lines being brought to a fine point, 

 are more easily positioned on a scale or pointer, wdiile the taper- 

 ing of faint or nebulous lines or bands enables their relative in- 

 tensity to be more easily compared. My experiment was a rough 

 one, and with a home-made slit ; but for working purposes, by 

 a sliding plate a slit might be contrived of a V shape, whose 

 widUi and length would be altered at pleasure; and it would, I 

 think, for some observations give good results. 



Guildford, Oct. 7 J. Rand Catron 



A Day Aurora 

 A controversv was carried on in your columns about a year 

 ago as to the possibility of an aurora being seen during the day 

 time. A recent convnuuication from Padre Secchito the French 



