COMETS, . 747 



Recent observations with the spectroscope seem to prove that a part of the 

 light from comets is really reflected sunlight, since it faintly exhibits a spectrum 

 like that of other bodies which shine by reflected light alone. 



The Polariscope (instrument for detecting reflected light) also gives evidence 

 which leads to the same conclusion. The use of this instrument in examining 

 faint sources of light is attended with great difficulties ; and in the present con. 

 nection, the records of different observers are not strictly harmonious. 



RECORD OF THE TELESCOPE. 



§ 3. Comets are seen in their simplest form as faint patches of nebulous 

 light. They are usually circular or oval in outline, without remarkable difference 

 of brightness from center to circumference. At a latter stage of development the 

 comet shows a diffused brightness in its central parts, known as central con- 

 densation. 



When a large comet approaches the sun, the structure becomes far more 

 complex. The center condensation gathers intensity. Finally, a point or disc 

 of light appears near its center, which shines with a light approximating that of 

 the planets. This is called the nucleus. 



NUCLEUS HAS NO DEFINITE MAGNITUDE. 



Observers with powerful telescopes usually find that what we commonly 

 call the nucleus has no definite magnitude. It continually measures less with in- 

 crease of optical power. The inference is that the real nucleus, if it consists of a 

 single solid body, must be very small, and much obscured by the vapors which 

 surround it. Generally the nucleus appears to shrink in size as it approaches 

 the sun. The most plausible explanation seems to be, that with lessened dis- 

 tance from the sun, the real nucleus gets hotter and brighter and at the same 

 time the vapors near it become more transparent. Other explanations have 

 been offered which we have no space to consider. 



THE ENVELOPES. 



We commonly find that the coma (nebulous matter about the nucleus) ap- 

 pears much brighter on the sunward side. In many cases, streams of matter ap- 

 pear to issue from the nucleus on that side. These assume a variety of forms, 

 and are almost always curved backward, from the direction of the sun, at their 

 extremities. Above the streams, or jets are sometimes seen one or more arcs 

 of light, concentric with the nucleus from which they appear to recede, — ^just as 

 waves recede in widening circles from a stone dropped into still water. These 

 envelopes, as they are called, are supposed to be hollow spherical segments of 

 matter, more dense than the surrounding parts of the coma. 



One highly important characteristic of the matter which surrounds the nu- 

 cleus is well established. The rays of light from distant stars seen through it are 

 not sensibly bent, or refracted. This shows that gases to an appreciable amount 

 do not exist in comets. 



