Intelligence and Miscellaneous Articles. 77 



a Orionis than in Arcturus ; and all are less so than in the sun. It 

 might, moreover, he asked whether even in our sun the intensity of 

 these lines is constant ; and this we are led to doubt when we com- 

 pare Fraunhofer's figure with that of Van der Willigen, in which 

 fainter lines are met with than those marked by the first observer. 

 The group which stands out the best is that of iron ; a Orionis has, 

 moreover, systems of gaseous absorption analogous to those of our 

 atmosphere. But this kind of investigation is only just sketched out, 

 and much time is required to arrive at definite results. 



Unfortunately this investigation cannot be accelerated, owing to 

 the small number of nights without any moon, and suitable for it 

 in other respects, and to the danger of fatiguing the sight too much. 

 A phenomenon is in fact frequently met with which shows how an 

 observer must be somewhat on his guard. When, after having 

 kept the eye fixed on a star for some time, we look into the finder 

 of the large telescope for several seconds, a perfectly dark band of 

 the same size as the spectrum is seen across the field itself. The 

 eye is thus paralyzed by the decomposed light far more than by a 

 white light of equal intensity. Inversely, on putting the eye to the 

 telescope, for the first few moments scarcely any of the numerous 

 details are observed, which begin to come out little by little until 

 complete definition is attained. These phenomena show that the 

 eye must be spared, and that without care it might be injured in 

 these researches. 



But at the same time we see what an intensity the system of cy- 

 lindrical ocular lenses can retain. Some time ago it could scarcely 

 have been suspected that the eye could be fatigued by the light of 

 the spectrum of a star. — Comptes Rendus, May 11, 1868. 



ON GILDING GLASS FOR THE PREPARATION OF OPTICAL MIRRORS. 

 BY W. WERNICKE. 



In the last ten years various methods of silvering glass for the pre- 

 paration of optical mirrors have been described, which have been 

 successfully applied in practice. In order also to enfeeble the action 

 of the sun's rays in solar observations, it is usual now to coat the 

 object-glasses of telescopes with thin layers of silver — a method first 

 proposed by Foucault, and now introduced into several observatories. 

 The nature of silver is opposed to a continued use of the metallic 

 mirror ; for when exposed to the air it loses its Lustre after no great 

 length of time, owing to the foreign gases present in the air. 



Hence it has been frequently proposed to replace silver by gold 

 for the above purposes ; but the methods published have turned out 

 to be of little use when tried. In vol. cxl. of Liebig's Annalen, 

 Liebig has described a method based on the reduction of a specially 

 prepared alkaline solution of gold by a mixture of alcohol and ether. 

 The success of this method depends, however, on points not fully 

 understood ; and it is therefore inapplicable on a large scale. 



