SCIENTIFIC NOTES. 141 



allowed for faults at the edge. For the two discs he is to receive 

 <£1,000, and has twelve months' time to make them in. It is 

 much to be regretted that some scientific Society, or individual 

 with time and means at command, does not take up the question of 

 progress in the manufacture of large telescope discs. I fear little 

 progress is to be expected while the manufacture continues as it is ; 

 and it is not surprising to me that so little has yet been done with 

 large refractors. The large one at Washington is in most able 

 hands, part of the time being devoted to satellites of Saturn, Uranus, 

 and Neptune, for which its great light-collecting power is most valu- 

 able. But though great part of the time is devoted to the exami- ■ 

 nation of the physical appearance of planets, &c, little or nothing 

 new has yet been seen ; indeed, the new dark line which I disco- 

 vered last year in the belts of Saturn with the HJ-inch Schroeder 

 telescope at the Observatory, had not been seen with it or indeed 

 with any telescope that I could hear of in Europe. With regard 

 to large reflectors, several experiments are in progress. One at 

 Edinburgh of 2 feet diameter and only 10 feet focus, is said to be 

 a failure, though beautifully made. The foci of the central part 

 and outside ring differ, and of course no sharp definition can be 

 obtained. The other one is the 4-feet silvered glass reflector, 

 projected by M. Foucault, and which he did not live to finish. 

 When I was at the Paris Observatory, early in July, the 

 polishing was finished, and they expected to have it silvered and 

 mounted on its fine equatorial stand by the 1st of August. They 

 were kind enough to set it up for me, and I had the pleasure of 

 seeing a clock face with it, and although the focus is only 28 feet, 

 the definition is very sharp and satisfactory. 



As to the possibility of constructing large achromatics with 

 success opticians differ in opinion widely. Some think it impossi- 

 ble to get good definition beyond about 12 inches diameter, simply 

 from the fact that the weight of the glass itself then becomes 

 sufficient to produce in some positions polarization. Others 

 think that the only limit to the size is the difficulty of making 

 the glass. 



Clocks, the next requisite for astronomy, are also under investi- 

 gation, and now that the mechanism is made so perfect that the 

 varying atmospheric pressure can be distinctly traced as a disturb- 

 ing cause, various devices are being resorted to for its removal. 

 Sir George Airy places the clock below ground with temperature 

 as uniform as possible, and makes the variation in the height of 

 the barometer alter the centre of gravity of the pendulum, so that 

 the clock rate is uniform. So far the result is very satisfactory. 

 Professor Forster, of the Berlin Observatory, shuts the clock up in 

 a glass cylinder, from which a part of the air is withdrawn, so 

 that there is a constant pressure on the ends of it which are 

 ground on. So satisfactory is this method of ensuring uniform 



