September 19, 1884. 



SCIENCE. 



295 



of search for new nebulae, and of simply recording 

 their approximate positions by pointing with unillu- 

 minated cross-wires in the eye-piece, and reading off 

 the circles of the instrument, recording with this a 

 description of the appearance of the nebula. His 

 reason for making no attempt to determine accurate 

 positions was that it would require illuminated mi- 

 crometer-wires, and a great deal of time devoted to 

 measurement with neighboring stars, besides much 

 time lost in letting the eye become sensitive again for 

 further search or examination after the light was re- 

 moved; he stating that his eye was practically 'nebu- 

 la blind ' for at least four minutes after being near a 

 light. Since, however, the most of these nebulae are 

 probably too faint to bear any illumination at all, and 

 must therefore be observed for position with ring or 

 bar-micrometer, much of this reason loses its force ; 

 for in this case there would be no loss of time on 

 account of light, and if in this way Mr. Swift could 

 connect each of these new nebulae to some neighbor- 

 ing star with the help of chronograph or an assist- 

 ant at clock or chronometer, and also re-observe the 

 known nebulae in the same way, the value of the 

 work would be almost immeasurably increased com- 

 pared with the little additional time and labor neces- 

 sary for its accomplishment. As it is, though no one 

 will deny the value of a catalogue of even the ap- 

 proximate positions and descriptions of very faint 

 nebulae, as a contribution to our knowledge of their 

 number and distribution, and as an aid in comet- 

 seeking or identification, yet it is fairly open to the 

 criticism, that, to be what it should be in the present 

 state of astronomical observation, it must all be gone 

 over again for determinations of accurate positions. 

 One very interesting statement of Mr. Swift, to the 

 effect that there had not been a first-rate clear sky 

 since the red glows appeared a year ago following the 

 Krakatoa explosions, bears out the general experience 

 of workers in other observatories, especially those 

 who try to see stars near the sun in the daytime. 



An interesting discussion arose as to the much-dis- 

 puted existence of the nebula round the star Merope 

 in the Pleiades; the general drift of it being that the 

 nebula no doubt existed, but in order to see it a clear 

 sky was necessary, and a very low power and large 

 field, so that the nebula might be contrasted with 

 darker portions of the same field; that a large tele- 

 scope was not necessary, in fact the smaller the bet- 

 ter, provided the optical qualities were relatively as 

 good. Mr. Swift said he could always see it under 

 favorable conditions; and Mr. E. E. Barnard of Nash- 

 ville, Tenn., the discoverer of the latest comet, said 

 that before he knew of its existence at all, he picked 

 it up as a supposed comet. 



On Thursday Professor Adams of Cambridge, Eng., 

 read a paper upon the general expression for the 

 value of the obliquity of the ecliptic at any given 

 time, taking into account terms of the second order. 

 The difficulties of obtaining a formula for this quan- 

 tity, on account of the many varying elements upon 

 which it depends, were clearly explained by a dia- 

 gram, and the results given of an approximation car- 

 ried much further than ever attempted heretofore. 



Professor Harkness, in paying a high compliment 

 to the celebrated mathematician and astronomer for 

 these laborious and valuable researches, also ex- 

 pressed a wish that some of the n-dimensional-space 

 mathematicians would follow the example of Profes- 

 sor Adams, and apply some of their superfluous en- 

 ergy to the unsolved problems in the solar system, 

 which have some direct practical bearing. 



Professor Newcomb, in remarking upon the mass 

 of the moon used in this problem, expressed the 

 opinion that this could be obtained most accurately 

 by observations of the sun, in determining the an- 

 gular value of the radius of the small circle described 

 by the earth about the common centre of gravity of 

 earth and moon, since this, in his opinion, seemed to 

 be the only constant which could be determined by 

 observation absolutely free from systematic errors, 

 and hence was capable of an indefinite degree of 

 accuracy by accumulated observations; and he asked 

 Professor Adams's opinion on this point. 



The latter replied, that he thought the quantity 

 too small for certain accurate determination, almost 

 beyond what could be actually seen by the eye in the 

 instruments used. 



Professor Newcomb admitted, in the case of ab- 

 solute determinations, the general impossibility of 

 attempting to measure what cannot be seen ; but, in 

 the case of differential or relative determinations in 

 which there was no supposed possibility of constant 

 , or systematic errors, he advanced the theory, which 

 he had thought of elaborating more fully at some 

 time, that such determinations might be carried by 

 accumulated observations to a sure degree, of accu- 

 racy far beyond what can be seen or measured by 

 the eye absolutely. 



Professor Adams hoped he would more fully elab- 

 orate and publish this idea, since there was in it an 

 element well worth careful consideration. 



Professor Harkness doubted the sufficient accuracy 

 of meridian observations of the sun, on account of 

 the distortions produced by letting the sun shine full 

 into the instrument; and spoke of the difficulties in 

 the transit-of- Venus observations from this cause. 



Professor Newcomb replied that he would have to 

 show that this would be periodic with reference to 

 the moon's quarters in order to affect this constant 

 systematically. 



Professor Adams then presented another note upon 

 Newton's theory of atmospheric refraction, and on 

 his method of finding the motion of the moon's 

 apogee. He described in an exceedingly interesting 

 manner how some unpublished manuscripts of the 

 great geometer had lately come into his hands at 

 Cambridge, which contained later work than is pub- 

 lished in the Principia. Space will not allow a de- 

 scription of the methods which these papers show 

 that Newton employed in attacking, and remarkably 

 successfully too, some of the problems which still 

 trouble astronomers to-day. Photographs of these 

 papers were exhibited, showing his wonderful neat- 

 ness and precise methods in computation. It was 

 something of a novelty to those gathered on this 

 occasion, to hold in their hands the facsimiles of the 



