National Academy of Sciences. 



17 



mlttcd that great reflating telescopes give very variable 

 results and are very apt to prove unsatisfactory As at 

 instance oi this, if we examine the record of Hersehel's 

 work, we find that nearly tho wholn of it was 

 done with Ms two-foot reflector; we shall almost arrive 

 at the conclusion that all the work accomplished with 

 the four-foot reflector might have been done with the 

 smaller instrument. The same comparison of results 

 leads us to a similar conclusion with regard to the four- 

 foot reflector of Lassoll probably tho largest ever con- 

 structed. He had under the clear skies of Malta made 

 many important observations; but when he took his 

 four-foot reflector there, hoping with it to verify his 

 discoveries, it does not distinctly appear that he suc- 

 ceeded. Struve, after looking through the four-foot tele- 

 scope, wrote that it was not in any remarkable 

 degree more powerful than his own 15-inch instru- 

 ment at Pulkova. The only exception to this gen- 

 eralization is the fact that the four-foot in- 

 strument of Lassell did really discover the two inner 

 satellites of Urauu?. Prof. Newcomb having redis- 

 covered these with tho new instrument, and thus veri- 

 fied Lassell's discovery, thinks that they could never 

 be seen with a 15-inch refractor. In the new telescope 

 the outer satellites of Uranus look as if of about the size 

 that d, JTrsce minoris appears to the. naked eye. Tne 

 smaller satellites, strange to say, have beeu best seen 

 when the moon was shining, and its light was plainly 

 apparent in the telescope; the first .of ttiese appears 

 about half as bright, and the second about one-third as 

 bright, as Titauia. 



It must be admitted that it is impossible to make a 

 refracting telescope perfectly achromatic. The second- 

 ary spectrum which is obtained is for certain kinds of 

 observations a serious objection to this class of lenses. 

 This is especially the case where an extremely faint ob- 

 ject has to bo observed alongside a very bright one. In 

 iDvestia-ating the working of all ordinary telescopes, if 

 we confine ourselves to the yellow and green rays, we 

 shall find the rays to be brought to very nearly the 

 same focus; but on exairiuins: tho other rays we find 

 that the red and the blue rays come to a longer focus, 

 while ttie focus for the extreme indigo and 

 violet rays is so much longer that they form a 

 halo around the star's image. Possibly this can be 

 avoided by adopting a device of the earlier astronomers 

 by having telescopes of one or two hundred feet in 

 length, and by making changes in the curves of the 

 glass. 



Tue difficulty in a refracting telescope is a theoretical 

 one; it is inherent in the instrument, and can never be 

 entirely avoided; that of a reflecting telescope is me- 

 chanical in its nature, but has hitherto proved the more 

 baffling of the two. On the whole, for regular work of 

 almost; every kind the refractor is beticr th.m tho 

 reflector. 



Our trier.ds have asked whether tbere is difficulty in 

 the Washington telescope on account of spherical aber- 

 ration. This proves to be a very small factor ; its total 

 amount is lees than that produced in thcleu.s bv ordinary 

 atmospheric variations of temperature an effect which 

 is noticed, when work is first begun with the instrument 

 of an evening, but which rapidly wears away as the 

 glass acquires the uniform temperature of the rest of 

 the instrument. It seems to bo only the rays -near the 

 edge of the glass which are thus affected. Prof. New- 

 comb has looked through many other refracting telc- 

 ecopes. by way of comparison, and after full considera- 

 tion he gives it as his unhesitating opinion that the new 

 instrument must bo regarded as a great success. 



Tlio following paper, in tho absence of its author, 

 was read by tho Secretary : 



THREE OF JUPITER'S SATELLITES. 



BY PROF. 8. ALEXANDER OP 1MJINCKTOX, X. ,T. 



It is claimed that the other sah-lliics of il 10 



planetary system resemble our moon in the coiiicidcne,, 

 of their times of rotation and revolution ; MIX! that in 

 consequence every satellite presents always nearly tho 

 same side to its primary. One occasion for this belief j s 

 found on observing the special vicissitudes whirl! ihn 

 light of the satellites exhibits, each spccili ,1 eimnge 

 recurring when they have again arrived at the sumo 

 position in their orbits around their respective 

 primaries. Another evidence is found in tho 

 remarkable phenomena of their apparent loss of light on 

 certain occasions. All Jupiter's satellites, except tho 

 second, have at times been seen when in transit on the 

 disk of the planet, appearing in whole or in part as dark 

 instead of bright spots; and sometimes after at first ap- 

 pearing bright they seemed to become dusky. This, as 

 Prof. Alexander has intimated in previous publications, 

 would seem to be due to the absorption of, and possibly 

 also, to the interference of light; i.e., of the light re- 

 flected from Jupiter meeting that of the satellite; and 

 all tnis on a scale such as is only seen in astronomical 

 observations. 



The extent of the undulations of light coming from the 

 planet should, it would seem, be greatest whore the pen- 

 etration through its atmosphere and the return are most 

 nearly in a vertical direction, i. e., near the middle of 

 the disk; while near its edges those undulations 

 traversing the atmosphere (both going and returning) 

 with great obliquity, would be more restrained. Ac- 

 cordingly a satellite may sometimes as it does appear 

 briiriit, possibly unusually bright, at its first entrance on 

 the disk of the planet. As it advances, under the par- 

 tial efivct of absorption, &c., it becomes dusky. Near 

 the middle of the transit it seems relatively black, con- 

 tinuing so sometimes to the end of t"ie transit, tho pas- 

 sage of the disk being, very possibly, in the retrion of a 

 bright belt. It is not strange, under these circum- 

 stances, that tho dark spot should not always be rouud. 

 Aside from all this, however, the phenomena in ques- 

 tion would seem to be consistent with the theory of a, 

 coincidence in the times of rotation and revolution, for 

 the appearance of the satellite in the course of its 

 transit as a black spot has within moderate intervals of 

 succession recurred when the satellite had returned to a 

 like position in its orbit around its primary. Admitting 

 tho absorption already indicated, then, instructed by 

 the revelations of the spectroscope, we may regard it as 

 possible that the satellite may be colder aian if s primary. 

 This would happenindeed we would have a reason for 

 t if the satellite, like the moon, had little or no at- 

 mosphere. All these analogies woukl IIP quite consistent 

 th the hypothesis that all the satellites (inclmlinsr the 

 moon) had been similarly condensed from the nebulous 

 state, and then subjected to tho stringent conditions 

 which prevail in satellite systems. 



The loss of atmosphere is one of tho suppo^able conse- 

 quences of those strineent conditions, as indued M. L'i- 

 )lace has intimated, when, afti-r stating the distance at 

 which the attractive force of the earth is in rqiiilibrum 

 with that of the moon, ho adds: " It at tins di.-tanco thu 

 iriuiitive atmosphere of the moon had not been deprived 

 jf all elasticity, it would be carried to the .-artii, which 

 would thus draw to itself (I'asjiircr). This is perhaps tho 

 reason why the rnoou'a atmosphere is nearly iuscusible." 



