ASTRONOMY AND METEOROLOGY. 405 



tion of the line of external attraction. As the body approached the sun, its 

 major axis would increase in length, and it would become more prolate; and 

 as it receded from the sun, the length of irs major axis would diminish, and 

 ic would approach more nearly to the spherical form. It now remains to 

 reconcile the appearances' usually presented by comets with the form which, 

 according to this view, they really possess. This Mr. Kemplay does by sup- 

 posing that we never see the whole of a comet (the gaseous matter of which 

 it is composed not being luminous in itself), but only such portions of it as are 

 illumined by the sun's rays; the apparent form of the comet being determined 

 l>y the refractive power of the matter of which it consists. The parallel rays 

 of the sun, falling upon the convex surface of the comet, will converge into a 

 focus, which will generally be within the spheroid, and will then diverge until 

 they reach its further limits. If the mass of the eornet be at all of a nebulous 

 or misty character, the course of the refracted rays will be indicated by a lu- 

 minous appearance; and the degree of brilliancy of any part (supposing the 

 mass to be uniformly nebulous) will depend upon the number of rays passing 

 through that part. The focus will be brightest, and this is the nucleus of 

 the comet; the head, or nebulous envelope of the nucleus, is formed by the 

 converging and diverging rays near the focus, and the tail by the continu- 

 ation of the diverging rays when they are further dispersed, and shine, con- 

 sequently, with a feebler lustre. 



Such is, briefly, Mr. Kemplay's theory. According to it the tail of a comet 

 ought always to extend in a straight line away from the sun, which is gener- 

 ally, but not always, the case. It explains the fact that the tail is longest 

 when the comet is nearest the sun. The spheroid becomes more prolate as 

 it approaches nearer to the sun, so that the surface on which the rays fall 

 becomes more convex as the comet approaches its perihelion; and the more 

 convex the surface, the shorter the focus, and consequently the longer the 

 diverging rays. As the comet recedes from the sun its surface becomes less 

 convex, and the focus gradually longer, until at last it falls beyond the limits 

 of the spheroid, and the comet is no longer visible. The sudden disappear- 

 ance of Halley's comet in 1836 may be accounted for in this way. The 

 backAvard curvature of the tail, which is frequently observed when the comet 

 is near its perihelion, is explained by the inclined position which the comet 

 assumes at that period, owing to its parts which are farthest from the sun 

 (ihe tail) having to pass through a much larger space in the same time than 

 the parts nearest the sun (the nucleus). The fact that the sides of the tail 

 are more brilliant than the centre, Mr. Kemplay attributes to the convergence 

 of more rays at the sides than in the centre, and illustrates his explanations 

 by diagrams. The streaming light which occasionally shoots along the tail 

 with inconceivable rapidity he accounts for by an undulation in the whole 

 mass of the comet, producing an effect analogous to that observed in a field 

 of corn when shaken by the wind. 



The above are the principal cometary phenomena of which Mr. Kemplay's 

 theory offers. a consistent explanation. Those which are inconsistent with it 

 are, principally, the occasional appearance of a shorter and wider tail, ex- 

 tending on each side of the original tail, and varying in position; the occa- 

 sional appearance of the tail at right angles to a line drawn from the sun; 

 and the fact that, when stars are seen through comets, no refraction of their 

 rays is observed. The first of these facts Mr. Kemplay can only account 

 for by supposing that the particular comets in which it Avas observed Avere 

 subjected at the time to some peculiar disturbing influence, which modified 



