C. GENERAL PHYSICS. 175 



are the longest lines observed in the spectrum of their va- 

 pors, the presence of these metals in the sun may be regard- 

 ed as established. 6 JB, LXXV., 1816. 



MENDING A BROKEN OBJECT-GLASS. 



It is not generally the case that astronomers attempt now- 

 adays to do delicate work with poor instruments, and we 

 should have thought that probably no living astronomer 

 would trouble himself to make a broken object-glass service- 

 able to science. The recent experience of Dr. Gould, in Cor- 

 dova, is quite remarkable in this respect. A letter received 

 from him not long ago announced that the large objective 

 sent him for use in stellar photography was found, on arrival, 

 to be broken into small pieces. The glass had been made 

 by Mr. Rutherford, of New York, for this particular class 

 of work, and there was but one similar to it in the Avorld. 

 The disappointment of those who had hoped that celestial 

 photography would become an important department of as- 

 tronomical Avork in the southern hemisphere is now happily 

 relieved by the announcement of Dr. Gould that he has so 

 far succeeded in patching together the pieces of the broken 

 objective as to afford reasonably distinct photographic im- 

 ages, and that he believes the instrument will yet prove 

 serviceable. The difficulty of replacing this glass by an en- 

 tirely new one was such as to quite justify the labor that Dr. 

 Gould has apparently bestowed upon his delicate task. The 

 photographic glass that he has now at his disposal has an 

 aperture of about sixteen inches, and a focal length of as 

 many feet. ______ 



ACTINIC POWER OF LIGHT. 



In confirmation of the views lately and very clearly set 

 forth by Professor Draper, must be noted the recent report 

 of a committee of the German Photographic Society on a 

 method of shortening the time necessary for the exposure of 

 the sensitive photographic plate by a previous or a subse- 

 quent exposure to diffused light. The experiments made by 

 this committee show that the time may be reduced by one 

 half, and that the slower the action of the extraneous light, 

 the better the result. The colored glasses of Gaensli were 

 found to give excellent results. These glasses, although they 



