1 50 Notices respecting New Books. 



may result from actual combustion going on in the higher regions of 

 their atmospheres, the elements of which, so united, may be in a 

 constant course of separation and restoration to their actual state of 

 mutual combustibility, by vital processes of extreme activity going 

 on at their habitable surfaces, analogous to that by which vegetation 

 on our earth separates carbonic acid (a product of combustion) into 

 its elements, and so restores their combustibility. With specific 

 hypotheses as to the causes of solar and sidereal light we have how- 

 ever no concern. It suffices that they must have a cause, and that 

 this cause, inscrutable as it may be, does in several cases, and there- 

 fore may, in one more, determine the production of phenomena of 

 the kind in question." — P. 352. 



The last part of the chapter is devoted to an account of some 

 attempts to compare the intensities of light of the stars one with 

 another by the intervention of the moon, by the aid of an astro- 

 meter, or instrument adapted to that purpose. " The process by 

 which these comparisons were made consists in deflecting the light 

 of the moon by total internal reflexion at the base of a prism, so as 

 to emerge in a direction nearly coincident with that of the undeflected 

 light of one of the stars to be compared. It is then received upon 

 a lens of short focus, by which an image of the sun is formed, which, 

 viewed at a considerable distance by an observer placed in or near 

 the axis of the lens, will appear to him as a star. This artificial star 

 is then to be approached to or removed from the eye until its light 

 is judged to be exactly equal to the light of the real star, which, 

 lying in nearly the same direction from the observer, will be seen 

 side by side with the artificial one with the same eye, or with both 

 eyes at once, without the aid of a telescope in the ordinary mode of 

 natural vision. The distance from the eye to the focus of the lens 

 being then measured, the prism and lens are to be so placed as to 

 form another similar artificial star, in a direction nearly coincident 

 with that of the other star under comparison ; and another equalisa- 

 tion being made, and distance measured, it is obvious that the intensi- 

 ties of the lights of the two stars, or at least their effects on the retina, 

 under the circumstances of comparison, will be to each other in the 

 inverse ratio of the squares of the distances so measured respectively. 

 For no light being lost by total internal reflexion at the base of a 

 prism, and the light lost at the ingress and egress of the moon's rays 

 into and out of the prism, being at such moderate angles of incidence 

 as it is ever necessary to employ in such comparisons, very nearly, 

 indeed, in an invariable ratio to the total incident light, the artificial 

 star, or lunar image, will be equally luminous in both cases, and its 

 effect on the eye will therefore be in the ratio of its apparent angular 

 diameter, or inversely as the square of its distance." — P. 353. 



The application of the principle explained in the above extract is 

 effected by a very simple construction. An upright pole about 20 

 feet high, firmly fixed in the ground as a standard, a staff consisting 

 of a cylindrical rod about 2f inches in diameter and 1 2 feet long, 

 and a slider consisting of a rectangular piece of deal about 1 5 inches 

 long and 2| inches broad, form its principal parts. The slider 



