202 ANNUAL OF SCIENTIFIC DISCOVERY. 



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the Munich, and vastly superior in convenience and value. The spring- 

 governor also was put into the best condition for good action there, the 

 heavy mass of the telescope acting directly as a balance wheel, and control- 

 ling all irregularity of movement. In short, the instrument satisfied all 

 reasonable requirements for equatorial mounting. 



KOBELL'S STAUEOSCOPE. 



At the German Association for the Promotion of Science, Dr. Grailich 

 exhibited Professor Kobell's Stauroscope, for the investigation of the optical 

 properties of mineral substances. The principle of this very simple apparatus 

 consists in the disappearance of the dark cross shown by plates of calcareous spar, 

 when placed between two tourmalines set across, by the interposition of any 

 crystallized substance whose principal section of elasticity is not coincident 

 with the polarizing planes of tourmaline. By turning the interposed plate of 

 crystallized substance, a position of it is found in which the principal sections 

 of elasticity coincide with the polarizing planes of the apparatus, and this 

 coincidence is manifested by the reappearance of the dark cross. 



GEOLOGICAL SPECIMENS FOE MICROSCOPIC PURPOSES. 



At the recent meeting of the German Association for the Promotion of 

 Science, Professor G. Eose exhibited Dr. Oschalz's collection of lamellas of 

 minerals and rocks, reduced to a degree" of thinness sufficient to render them 

 transparent, and fit for microscopic investigation. This mode of investigation 

 has led to the observation of many interesting facts respecting the crystalline 

 structure of simple minerals, and the real constitution of composite sub- 

 stances, such as basalt, solenite, porphyry, &c. These plates, overlaid with 

 Canada balsam, are placed between two pieces of glass, so as to be easily 

 brought under the microscope. The price of the set of seventy-three plates 

 is thirty-five thalers (about 51. 8s.). 



ON THE PERMANENCE OF PHOTOGRAPHIC PICTURES. 



The Committee of the English Photographic Artists, appointed to consider 

 the question of the fading of photographic pictures on paper, have submitted 

 a first report as follows : 



The committee have unquestionable evidence of the existence of photo- 

 graphs which have remained unaltered for more than 10 years, prepared by 

 salting plain paper with a chloride, afterwards making it sensitive with either 

 nitrate or ammonio-nitrate of silver, fixing with a freshly made solution of 

 hyposulphite of soda, and washing with water ; also of positives produced by 

 Mr. Talbot's negative process. 



They have not been able to obtain evidence of photographs having been 

 prepared at all upon albumenized paper, or colored with a salt of gold, or 

 fixed 'with "old hypo," so long as ten years. They have, however, ample 

 evidence of the existence of unaltered photographs so prepared five, six, and 

 seven years ago. 



They have not found that any method of printing wliich has been commonly 

 followed, will necessarily produce fading pictures, if certain precautions bo 



