460 Royal Astronomical Society. 



creases the irradiation, but at the same time it diminishes the light. 

 At a certain point, then, these two causes counterbalance each other, 

 and no further enlargement takes place. This limit will vary with 

 each instrument, and we have no certain grounds on which to de- 

 termine it. Various observations are referred to in which its influ- 

 ence is evinced. 



The astronomical facts connected with these causes are then ex- 

 amined from the testimony of various observers. In particular the 

 application of these principles to some of those singular phaenomena 

 occasionally noticed in eclipses, transits, occultations, &c. seems easy 

 in theory abstractedly considered. The difficulty lies in explaining 

 why they are observed only in some cases and not in others. The 

 author dwells particularly on the desirableness of a closer attention 

 to stating all the conditions of the telescopes employed, especially the 

 apertures. 



In particular the phsenomenon " the necic," in the transits of Mer- 

 cury and Venus, would be an obvious consequence of irradiation, 

 which would diminish the planet's disc and enlarge that of the sun 

 except at the small portion of the circumferences in contact, when 

 the absence of both irradiations would produce a " neck." 



Both theory and experiment show that a small dark disc would 

 have for its image a diminished disc with a bright internal concentric 

 ring, which, if the disc be very small, will be contracted to a central 

 bright point. This seems to agree with the appearance noticed by 

 several observers in the transit of a white spot on the centre of the 

 planet. On a former occasion, however, Professor Moll and others 

 saw such a spot excentrical. The projection of a star on the bright 

 limb of the moon would also be an effect of irradiation, which would 

 cause the disc of the moon simply to overlap the star. 



Lastly, the author suggests a method for obtaining measures of the 

 amount of irradiation under any given light, by placing a card, cut as 

 before, at the focus of a lens, opposite to the object-glass of a tele- 

 scope, and attached to it by a short tube ; when the enlargement of 

 the image of the card, illumined by the light from any source, can 

 be subjected to the exact measurement of the micrometer of the 

 telescope. 



On a New Method of Observing Transits. By A. D. Bache, Esq., 

 Director of the American Coast Survey. 



" Permit me to invite your attention, and that of the members of 

 the Royal Astronomical Society, to a brief abstract of an official 

 report made to me on the 15th inst. by Mr. Sears C. Walker, one 

 of the assistants of the United States Coast Survey, It relates to 

 the printing, by the use of an electro-magnetic clock, in connexion 

 with Morse's telegraph register, of the actual dates of any celestial 

 phsenomena, which are ordinarily made the subject of observation 

 by astronomers. 



"The electro-magnetic clock of Mr. Wheatstone is described in the 

 Proceedings of the Royal Astronomical Society for Nov. 19, 1841. 

 Mr. Steinheil has described his in Schumacher's Astronomical Jahr- 

 buch for 1844. 



