EECEEATIVE SCIENCE. 



195 



it are reflected, which reflection varies in 

 intensity with the colour of the reflecting 

 surface. The reflection from white surfaces 

 is the brightest, because here the greatest 

 number of rays is reflected, and its inten- 

 sity diminishes as the proportion of those 

 absorbed increases. 



The reflection of non-luminous bodies, 

 which characterizes polished surfaces, may, 

 perhaps, be explained, when we consider that 

 the asperities of such are far more reduced 

 than those of rough objects. When light 

 falls upon rough or unpolished surfaces, as a 

 piece of slate, it is easy to conceive that the 

 rays, before quitting them, are many times 

 reflected, which, of course, renders them 

 feeble, and thus unable to return the impres- 



sions of objects in front. Consequently, if 

 this explanation is accepted as true, the dif- 

 ference of reflection from polished and rough 

 surfaces is merely one of degree, although ap- 

 parently of kind. The reflection of light from 

 unpolished surfaces only suffices to produce 

 blank colour, showing nothing beyond. I 

 consider this hypothesis strengthened by the 

 fact, that unpolished surfaces will, although 

 not very distinctly, reflect luminous bodies ; 

 a wall will reflect the image of a candle if 

 placed pretty close to it, from which it ap- 

 pears probable that the strength of the rays 

 overcomes the many reflections, and thus, for 

 the time, causes a rough surface to act as a 

 polished one. 



J. A. Davies. 



A CATALOGUE OF ALL THE COMETS WHOSE CEBITS HAVE HITHEETO 



BEEN COMPUTED. 



A NEW comet having been discovered, the 

 tirst thing an astronomer does, is to obtain 

 three observations of it, whereby he may 

 compute the elements of the orbit. He then 

 examines a catalogue of comets, to see if he 

 can identify the newly-found stranger with 

 any that have been before observed. The 

 value of a complete catalogue is therefore 

 obvious, and as nothing of the kind has, as far 

 as the writer is aware, been published for 

 some years, he has been led to compile a 

 new one. 



In the preparation of the following, care 

 has been taken that only the most reliable 

 orbits that were to be obtained should be in- 

 serted, the general rule being to prefer the one 

 which was derived from the longest arc, other 

 things being satisfactory. Among the autho- 

 rities consulted may be mentioned Pingre, 

 Hussey, Olbers, Cooper, Hind, Arago, and 

 others. 



From the Journals of the Eoyal Astro- 



nomical Society of London, the Academy 

 of Sciences of Paris, the Astronomischo 

 Nachrichten, etc., much valuable informa- 

 tion has also been obtained. 



PP denotes the time of perihelion pas- 

 sage expressed in Grreenwich mean time, N.S., 

 since 1582. 



TT denotes the longitude of the perihelion. 



U denotes the longitude of the ascending 

 node. 



I denotes the inclination of the orbit to 

 the plane of the ecliptic. 



q denotes the perihelion distance ex- 

 pressed in semi-diameter of the earth's orbit. 



6 denotes the eccentricity (of an elliptic 

 orbit). 



M denotes the direction of motion, -f- 

 direct, — retrograde. 



The periods assigned in the column of 

 " duration of visibility" are subject to much 

 uncertainty in the case of the ancient 

 comets. 



