May 15, 1884] 



NA TURE 



53 



angle of which varies, and it becomes almost impossible in a 

 broken equatorial, in which the eyepiece is independent of the 

 moving part of the instrument, as proposed by Mr. Grubb. In 

 a word, taking no account of the new and very grave causes of 

 variability introduced by Mr. Grubb, this optical system is so 

 unstable that its employment has been rejected unanimously by 

 all astronomers and opticians. The least derangement of the 

 position in the central mirror spoils everything. 



The mobility of the plane mirror presents equally, from the 

 optical point of view, a slight inconvenience. The quantity of light 

 varies with the different angles of inclination, which renders the 

 I exact researches of the photometric very difficult. Without making 

 I one feel all its gravity, Mr. Grubb has, in truth, indicated the 

 defect of this optical system. But in order to turn the difficulty 

 lie suggests that, since the field of view becomes smaller as 

 the instruments become larger, we may content ourselves with 

 observing at a central point. But this is an affirmation pure and 

 simple. It is necessary in many measures of precision to have a 

 large field of view. The contrary will present several serious ob- 

 jections. We have, in fact, to observe stars in relation with other 

 stars, to measure, forinstance, the difference of declination between 

 a planet and a star of comparison. But we cannot make both 

 these observations at the centre. The same thing will always be 

 happening, in the case of comets, nebula, and clusters. It seems 

 to me, on the contrary, that a telescope is more perfect the larger 

 the field of view. Feeling thus, I have had made by Prazmow- 

 ski, for my new equatorial coude, achromatic eyepieces giving 

 a very large field. For the observation of comets I have such 

 an eyepiece, which magnifies fifty times and has a field of view 

 such that I can observe a degree. For a telescope of twenty-seven 

 inches we might have such an eyepiece with a field of twenty-four 

 minutes. 



From all which precedes, I think everybody will agree that 

 the system proposed by Mr. Grubb is far inferior to that now 

 employed in ordinary equatorials. 



(2) The mechanical point of view. The instrument consists 

 actually of an ordinary equatorial, in which the part which carries 

 the eyepiece is replaced by a counterpoise. It presents therefore, 

 from the point of view of stability, all the defects of the ordinary 

 instrument. Additional causes of instability inherent to the 

 design are — 



The micrometer and eyepiece are completely independent of 

 the principal mass, which necessarily gives rise to different defects 

 of decentering between the separated parts. Moreover, in this 

 instrument there are three distinct movements. In addition to 

 declination and right ascension, there is a third, which consists of 

 a differential movement round the axis of the mirror. This last 

 one constitutes alone, from the point of view of stability, a com- 

 plication which does not exist in the ordinary instruments. All 

 the movements of transmission are broken at a right angle, and 

 are four times as long as those of an equatorial coude of the 

 same size. There will therefore be such loss in transmission, 

 one would never have the instrument perfectly adjusted and 

 oriented. We see, from the mechanical point of view, there is 

 such an incoherence between the different parts of the apparatus, 

 that it is inferior to those now employed. And it really can- 

 not be compared with mine, which is almost as stable as a 

 transit instrument, and in which the movements of transmission 

 are excessively simple. 



(3) If we examine Mr. Grubb's instrument from an astro- 

 nomical point of view, we see it is based on a principle which 

 no astronomer can admit, namely, that it is superfluous to ob- 

 serve the greater part of the northern heavens. In many investi- 

 gations among the most elevated in the astronomy of precision 

 — let us take stellar parallax as an example — one is obliged to 

 combine observations made at different epochs of the year, and 

 it is only by the combination of measures thus obtained that the 

 desired result is arrived at. These stars must be observed, 

 therefore, in the northern part of the heavens as well as in the 

 others, for the vicissitudes of climate do not permit the astrono- 

 mer to observe exactly how and when he wants. The same 

 necessity presents itself in the study of the double stars ; to 

 ascertain and to discard the systematical errors in the angles of 

 position the astronomer is obliged to observe these stars in all 

 the celestial regions. If one wishes to limit one's self to the 

 exploration of one side of the heavens, one would lose precious 

 opportunities and gratuitously introduce serious difficulties. 

 There are also many cases in which this choice is not possible. 

 Thus, if we wish to discover new comets every part of the heavens 

 must be explored, and if one wishes to observe them they must 

 be observed where they are. 



Finally, permit me to ask Mr. Grubb how he is going to study 

 that part of the heavens which lies between 20° from the zenith 

 and the Pole. This region of space, I take it, would be entirely 

 closed to the observer with Mr. Grubb's arrangement. Any 

 research, therefore, which touched the stars covering this large 

 area could not be undertaken. 



The independence of the micrometer of the rest of the 

 instrument renders impossible any measures of precision. The 

 orientation of the micrometer, in fact, is the fundamental base of 

 every measure, and to do this preliminary work properly three 

 or four successive operations have to be performed, and take the 

 mean of the readings and adjust the apparatus by means of the 

 circle of position. But this fundamental operation cannot be 

 performed on Mr. Grubb's instrument. In fact, in practice, if 

 one wished to take an angular measurement with this instrument, 

 one would have to proceed somewhat in this wise : First of all it 

 would be necessary to content one's self with one approximation as 

 to the orientation ; then to repeat this after every individual 

 measure ; and lastly to take into account the disorientation of 

 the micrometer, to submit the readings of the circle of position 

 obtained to fastidious computations with a view to compen- 

 sate them. This gives an idea of all the inextricable com- 

 plications in which one would find one's self involved in this 

 case. In fact, to secure a simple observation of a comet it would 

 be necessary to increase the readings and the calculation by four 

 times, and after all one would only get a result inferior to that 

 furnished by an ordinary equatorial. I don't believe there is a 

 single astronomer in the wide world who would undertake 

 observations of precision under such conditions. 



It is quite true, as Mr. Grubb indicates, that the oculaire 

 might be connected with the rest of the instrument, but then, new 

 inconveniences of another order would arise. These, however, 

 I will not discuss now, for, as I said at the beginning, Mr. Grubb's 

 actual proposal is now alone in question. However this may be, 

 I consider the conception of this equatorial is so defective, taken 

 as a whole, that I do not think its adoption would be seriously 

 recommended. Nor do I think that the project will go beyond 

 its present stage, unless essential modifications are introduced, 

 and in this case the instrument would become like my own. 



Paris Observatory M. LcEWY 



Dust-Free Spaces 



I venture to call attention to some points in connection with 

 the observations on "dustless spaces," &c, as detailed in the 

 report of Dr. Lodge's lecture published in Nature, vol. xxix. 

 p. 610. 



Certain observations and studies of my own lead me to think 

 that, if attention be given to the points to which I wish to call 

 the notice of physicists, results of the highest importance may be 

 reached by means of the method of experimenting developed by 

 Dr. Lodge and Mr. Clark, and described in the report referred 

 to. 



Dr. Lodge's statement (p. 611) that "cloud spherules are 

 falling, but falling very slowly," is true when these spherules 

 are not at a higher temperature than the atmosphere in their 

 neighbourhood. When, however, very small particles floating 

 in the air become heated, they warm the air immediately sur- 

 rounding them, and then these particles are either buoyed up by 

 a small envelope of heated and dilated air clinging to their sur- 

 faces, or they are borne aloft by the local currents which they 

 create by contact with the surrounding atmosphere. 



Observations continued for nearly fourteen years have con- 

 vinced me that in ordinary clouds these two methods of lifting 

 are combined — that to a certain extent each of the spherules 

 or very many of the spherules of clouds are buoyed by adherent 

 heated and dilated air, and that the whole of the cloud, in many 

 cases at least, becomes warmer than its neighbourhood in general, 

 which adds to its buoyancy as a mass of intermingled air, water, 

 and vapour. 



These remarks apply also to small particles of matter other 

 than water. The action is the same except in degree. The 

 very high specific heat of water enables it to heat surrounding air 

 more readily and quickly than other substances do, and as a 

 consequence masses of water as in clouds are lifted more quickly 

 and to a greater height than masses of other bodies having the 

 same proportion of surface to weight. 



If it be remembered that radiant heat passes uninterruptedly 

 through air, i.e. that air is diathermous, it will be seen that 

 radiations from a distance striking upon particles of athermanous 

 bodies suspended in the air will cause these latter to heat the 



