66 Remarks on the Tails of Comets. 



steady light, or remain at once luminous and stationary, in any 

 form in the pure air."* 



So much for the theory of Hamilton, and the reasoning of our 

 author, whose conjectures must be established before they can be 

 entitled to the character of objections. There are moreover some 

 difficulties which have presented themselves to my own mind, 

 and may also have been noticed by others. Thus, one would 

 suppose that the bending of the tail towards the region which 

 the comet is leaving, might be more than adequate to answer the 

 end of aberration ; but in estimating this, the position of the tail 

 relative to the observer, as well as its length, must be taken into 

 the account. When the tail lies oblique to the line of vision, the 

 extremity may be many millions of miles more remote than the 

 nucleus, and some minutes of time may elapse after the arrival of 

 the light from the nucleus, before that from the remoter parts of the 

 tail reaches the earth ; hence the interval during which the comet 

 moves on in its course, is very much augmented, and the result 

 is a corresponding increase of curvature in the tail. 



Again, there has sometimes been observed a general obliquity 

 of the tail to the prolongation of the radius vector of the comet, 

 when near its perihelion, amounting to some degrees. When we 

 consider the great difficulty of obtaining a correct measure of 

 this deviation, arising from the peculiar circumstances under 

 which the comet is often seen when near its perihelion, this diffi- 

 culty also vanishes. In the first place, when these angles have 

 been measured, the unerring laws of perspective seem to have 

 been wholly disregarded. The unequal effect of refraction also 

 upon the nucleus and the extremity of the tail when near the ho- 

 rizon and oblique to it, an effect of no small consequence, has 

 been subject to similar neglect. In the second place, very many 

 of the visible comets on which observations have been made 

 when near their perihelia, have been visible only when near the 

 horizon, from the very fact of their proximity to the sun, and like 

 other heavenly bodies, have been occasionally subjected to dis- 

 tortion beyond the ordinary effect of refraction, rendering accu- 

 rate measurement altogether impracticable. Indeed, the whole 

 train of observations on the tails of comets, seems to have been 

 made with little or no reference to the ordinary influence of the 



* See this Journal, Vol. xix, No. 2. 



