178 Silvered Mirror Telescopes. 



cell, and keep it in a box when out of use, in a dry, cool 

 place. 



We left a mirror in our telescope all through the damp, 

 foggy, and rainy weather of last year, protected first by a 

 cover over the mouth of the tube, and secondly by an American 

 cloth case over the instrument, and very little harm was done 

 to the silvering. During a series of thick murky fogs we left 

 some pieces of silvered mica freely exposed in the observatory, 

 and they soon exhibited rainbow colours, while the protected 

 mirror displayed no tarnish. Constantly taking a mirror in 

 and out is very troublesome, and has the disadvantage that the 

 instrument is not ready for those sudden observations that are 

 often successful in changeable weather. We were, therefore, 

 anxious to find out what would be the result of the treatment 

 mentioned, and it seems that a mirror so circumstanced and 

 freely used on damp nights, if anything is visible, will keep 

 its lustre with an occasional rubbing for a considerable time. 

 Ours was very little the worse for a year's use, and would pro- 

 bably have lasted at least a year longer, but for a special pur- 

 pose we washed the silvering off. 



Having experimented on silvering these glasses, we can 

 affirm that no one accustomed to chemical processes need be 

 afraid of failure, though the best possible silvering, perfectly 

 free from specks, is of course a work of skill. The probable 

 defect of amateur silvering will be the occurrence of a few 

 spots and specks quite unimportant to optical performance. 

 There may also be a tendency of some parts to be weaker than 

 others, and to give way first after a succession of rubbings. 

 It is essential that no rubbing shall ever be administered unless 

 the mirror is perfectly dry. If wet or damp, off comes the 

 silver with a touch, though when dry it may be rubbed like 

 a spoon. • 



It is useless to try the silvering process in very cold 

 weather. A warm day is clearly the most favourable ; excess 

 of cold prevents the adhesion of the film. The re-silvering 

 process is cheap, whether done by the amateur, or performed 

 for him by Mr. With, and when the mirrors are packed in 

 suitable boxes, they can travel without risk. 



When the silver begins to go, fine cobwebby cracks 

 appear, but they do no harm for many months ; and even in 

 the atmosphere of Birmingham, with its thousands of chimneys 

 pouring out all imaginable smokes and vapours, Mr. Bird 

 finds the silvering stand for a considerable time. London 

 atmosphere, as we have found, is far less destructive than 

 might be expected. 



Among the disadvantages of the silvered mirror telescopes, 

 must be reckoned that plague of all reflectors, chimney cur- 



