Comets. 305 



reflector, bull's-eye, etc. Eemove A, and open the slit by 

 means of the milled head, not shown in cut, but which is at 

 right angles toDD. When the slit is sufficiently open the 

 rest of the apparatus acts like an ordinary eye-piece, and any 

 object can be focussed in the usual way. Having focussed the 

 object, replace A, and gradually close the slit till a good spec- 

 trum is obtained. The spectrum will be much improved by 

 throwing the object a little out of focus. 



Every part of the spectrum differs a little from adjacent 

 parts in refrangibility, and delicate bands or lines can only be 

 brought out by accurately focussing their own part of the 

 spectrum. This can be done by the milled head B. Dis- 

 appointment will occur in any attempt at delicate investiga- 

 tion if this direction is not carefully attended to. 



This spectroscope can be fitted so as to go under the stage 

 of any microscope, but it is probable that it will be little used 

 in this position, as for nearly all experiments the mode described 

 is more convenient. When the spectra of very small objects 

 are to be viewed, powers of from ^ inch to 3 - th, or higher, 

 may be employed. 



Blood, madder, aniline red, permanganate of potash solu- 

 tion (quite fresh) are convenient substances to begin experi- 

 ments with. Solutions that are too strong are apt to give dark 

 clouds instead of delicate absorption bands. 



Mr. Browning makes small cells and other contrivances to 

 hold fluids for examination. 



COMETS. 



AN ACCOUNT OE ALL THE COMETS WHOSE ORBITS HATE NOT BEEN CALCULATED. 



BY G. F. CHAMBERS. 

 {Continued from Fage 226.) 



1518. During the nights preceding April 6, a pale comet 

 was seen above the citadel of Cremona. — (Cavitellius, Annales 

 Cremonenses.) 



1520. In February, a comet appeared. — (Biot.) 



1521. In April, a comet with a short tail appeared in the 

 latter part of Cancer. — (Vicomercatus, Lubienitzki.) 



1522. A comet was seen in the W. — (Mizaldi, ii. 11.) No 

 month given. 



1523. In July, a comet was seen near a Ophiuchi. — (Biot.) 

 1529. In February a long star traversed the sky. This 



phenomenon renewed itself in August. — (Biot.) European 



VOL. VIII. — NO. IV. x 



