Mr. Rogers on Achromatic Telescopes. 381 



purpose. One advantage, however, might be obtained by 

 giving up this facility of adjustment ; for by making the interior 

 surfaces of the correcting lens to coincide, and by cementing 

 them together, we could prevent all loss of light by reflection 

 from these surfaces. Nor does the construction appear, even 

 with this limitation, so difficult as the ordinary one ; since the 

 alteration of one surface only, namely, the second of the con- 

 cave lens, would be necessary. If the curvature of this sur- 

 face were too great, the surplus of concave aberration would 

 have to be removed by flattening it, instead of separating the 

 lenses. 



Alexander Rogers. 

 Leith, 28th October, 1829. 



Chemical Examination of a Native Arseniuret of Manganese. 

 By Robert John Kane, Member of the Meath Hospital 

 Medical Society, Dublin. 



Sometime since I obtained from a person, who had a collec- 

 tion of minerals for sale, a number of specimens of various ores 

 of manganese, amongst which was one ticketed " Manganese 

 Ore, Saxony." It was sold me as a specimen of the native 

 peroxide. It was rather a small piece, about two and a half 

 ounces weight, and being based on a mass of foliated galena, 

 was pierced, in every direction, by small veins of ferruginous 

 quartz. I did not take much notice of it at the time, but 

 some months afterwards, being desirous of comparatively ex- 

 amining the different specimens of the native oxide which I 

 possessed, I exposed, in a tube retort, to a red heat, a few 

 grains of this mineral. No oxygen came over, but I was much 

 surprised to see that a substance rose in vapour, and con- 

 densed under the form of brilliant acicular crystals on the in- 

 side of the cool portion of the tube; I immediately removed 

 the retort from the fire, and in so doing, it cracked. The 

 external air got admission, and the mineral inflamed, burning 

 with a blue flame, throwing off copious white vapours, and 

 emitting a strongly alliaceous odour. 



I was surprised at this phenomenon, as there had been no 



2C2 



