652 Chronicles of Science. [Oct., 



upon but three stars as tests, and they too often obscured by bad 

 weather. The mode of correction was almost entirely of a local 

 nature, such as practised by the late Mr. Fitz and Mr. Clark for many 

 years. 



The objective was completed about December last, and the correc- 

 tions of it are such that it is thought to be capable of depicting any 

 object as seen, provided there be sufficient light, and no atmospheric 

 obstacles. As respects the light ; the objective has given photographic 

 images of stars, designated by Smyth as of the 85- magnitude, and 

 other stars on the same plate of full a magnitude lower. In the 

 cluster Proesepe, within the space of one degree square twenty-three 

 stars was taken, many of which are of the ninth magnitude, with an 

 exposure of three minutes. An exposure of one second gives a strong 

 impression of Castor, and the smaller star is quite visible with half a 

 second. With an achromatic objective it was necessary to expose 

 Castor ten seconds to obtain a satisfactory result. 



The great obstacle which prevents the results of photography from 

 realizing the achievements of vision is atmospheric disturbance. In 

 looking at an object the impression is formed from the revelations of 

 the best moments, and it is often the case that the eye can clearly 

 detect the duplicity of a star, although the whole object is dancing 

 and oscillating over a space greater than its distance. The photograph 

 possesses no such power of accommodation, and the image is a mean 

 of all the conditions during exposure. It is, therefore, only on rare 

 nights in our climate that the picture will approach the revelations of 

 the eye. 



Up to the date of the author's paper only one night had occurred 

 with a fine atmosphere, and on that occasion the instrument was occu- 

 pied with the moon ; the negatives were described by the author as being 

 remarkably fine, and superior in sharpness to any he had yet seen. 

 The exposure for the phase three days after the first quarter is from 

 two to three seconds, and for the full moon about a quarter of a second. 

 Its powers have not yet been tested upon the close double stars, 2" 

 being the nearest pair it has been tried upon. This distance is quite 

 manageable, provided the stars are of nearly equal magnitude. The 

 power to obtain images of the ninth magnitude stars with so moderate 

 an aperture, promises to develop and increase the application of pho- 

 tography to the mapping of the sidereal heavens, and in some measure 

 to realize the hopes which have so long been deferred and disappointed. 



It would not be difficult to arrange a camera-box capable of ex- 

 posing a surface sufficient to obtain a map of two degrees square, and 

 with instruments of large aperture we may hope to reach much smaller 

 stars than have yet been taken. There is also every probability that 

 the chemistry of photography will be very much improved, and more 

 sensitive methods devised. 



Proceedings of the Eoyal Astronomical Society. 



Professor Wolf has presented a paper on the course of the sun-spot 

 phenomenon, in which he discusses the statements made by himself 



