HINTS TO OBSERVERS OP HALLEY*S COMET. 135 



the notice of the astronomical readers of the Analyst, in the hope that, by 

 their united efforts, they may be able to ascertain some of them satisfactorily. 



In the first place, however, it may not be amiss to premise, to unpractised 

 observers, a few hints on the management of their telescopes. No observation 

 should be made with an erecting or terrestrial eye-piece, as the additional 

 number of lenses occasions a great loss of sight : astronomical eye-pieces alone 

 should be used ; or a terrestrial eye-piece may be converted into an astrono- 

 mical one, by unscrewing the tube nearest to the eye, and taking out the two 

 lenses from its interior end ; as the field of view may, however, in this case, 

 be rather unserviceable, it will be a still farther improvement to remove the 

 innermost of the two lenses at the eye-end, leaving only two lenses in the 

 whole instrument (if an achromatic), viz : the object glass, and that next the 

 eye. In this case, the edges of the field will be obscure, and its appearance 

 not satisfactory to an eye unaccustomed to it, but an object in its centre will 

 be seen very clearly and distinctly in an inverted position. The focal 

 length of thetelescope, it should be observed, will be greatly shortened by 

 this adjustment, which, if previously untried, should be made, at first, upon 

 terrestrial objects in the day-time. If the instrument has several astronomi- 

 cal eye-pieces, it will be proper to use them all, in any observation that is in 

 the least doubtful ; but the higher powers will be found deficient in light, in 

 observing so faint a body ; and, generally speaking, no higher power should 

 be employed than is sufficient to develope satisfactorily the phenomena un- 

 der examination : the advantage of a large field will be found to be very 

 great in ascertaining the extreme boundaries of an object whose termination 

 is frequently so difficult to be distinguished ; and from the neglect of this 

 precaution astronomers of eminence have occasionally been led into consider- 

 able errors. It may not be so generally known as it deserves to be, that the 

 deposition of dew upon the object-glass may be entirely prevented by the 

 employment of a tube of stiff paper or pasteboard, made to fit the end of the 

 instrument ; this not only saves much time, but obviates the necessity of 

 wiping the object-glass, an operation seldom performed without some danger 

 of scratching it. If, in addition to this precaution, the brass cap is placed 

 upon the glass, before it is removed into the warm air of the house, damp 

 will scarcely ever be formed upon it under any circumstances. 



The particulars which it is desirable to ascertain, may be enumerated as 

 follow : — 



1. Whether the Comet has anything that can be called a moderately well- 

 defined disc in the centre, or whether it is a mere luminous point, or only a 



general and gi-adual accumulation of fight, without any apparent outline 



The former, though not very usual, was certainly the case with this Comet 

 at its return in 1682, when it is described, by Cassini, as exhibiting in the te- 

 lescope a disc " aussi rond, aussi net, et aussi clair que celui de Jupiter.* 



2. If it has a disc, whether that disc has an uniform illumination, oris 

 brightest in the centre ; and Avhether its appearance is invariable, in this re- 

 spect, at different times. 



3. Whether the disc at any time exhibits any spots, or anything of a mot- 

 tled aspect, or is irregularly terminated, or seems to be composed of an as- 

 semblage of small fragments ; and, in this case, whether their relative bright- 

 ness and position are subject to any change. 



4. Whether the disc is accurately circular or not. In 1682 the nucleus, 

 according to Hevelius, had throughout an oval form,-|- but this appears to be 

 contradicted, by Cassini, in the passage already cited ; and it is not recorded 

 to have been noticed at its last return in 1759. (The previous returns, it 

 will be remembered, were anterior to the employment of telescopes.) 



6. Whether the disc, as it approaches the perihelion, manifests any symp- 

 tom of phases. This most curious feature was actually exhibited by this 

 Comet in 1682, according to Delambre. He states that Picard and La Hire 



* Memoires de PAcademie iiot/a/e, 1699. 



+ Littrow, Beytrage zu einer Monographic des Halley^s chen Cometen, Wien, 1834, p. 56 



