ARTICLES 25 1 



levers with flat plates at one end which support the mirror 

 evenly over its surface and counterbalance its weight at all 

 elevations. The edge support consists of a counterpoising ring, 

 bearing along the neutral plane of the mirror, the ring being 

 pressed against the mirror by a number of short weighted levers. 

 This type of support was designed by Prof. Ritchey for the large 

 reflector at the Yerkes Observatory and has proved extremely 

 efficient. An elaborate water-cooling arrangement surrounds 

 the mirror to maintain it at a constant temperature. 



The rotation of the dome, the opening and closing of the 

 shutters, and the setting of the telescope in right ascension and 

 declination are all electrically controlled, so that the manipula- 

 tion of the instrument in spite of its large size is very simple. 



At the time of writing, regular observations have not been 

 started with the instrument. Before these can be commenced 

 there are many small adjustments to be made, and slight modi- 

 fications of various parts will doubtless be found necessary. 

 After these are completed the details of the performance of the 

 telescope will be awaited with interest. It may be asked, Of 

 what advantage is so large an instrument ? Its main import- 

 ance lies, of course, not only in its large light-gathering power, 

 which will enable much fainter objects to be studied than would 

 be possible even with the 60-inch telescope, but also in its 

 large resolving power, which will enable very close objects to be 

 separated. In photographic work, the impression on the plate 

 is cumulative, and by giving very long exposures photographs 

 of very faint objects may be secured. But there are limits 

 to the possible length of exposure, for the background of the 

 sky is never absolutely dark, and too long an exposure leads to 

 fogging of the plate. No increase of exposure time will, how- 

 ever, increase the resolving power of the telescope, although it 

 may enable fainter detail to be seen. The advantage of the great 

 resolving power would be lost if the observing conditions were 

 not extremely' favourable, and in that respect the Mount 

 Wilson Observatory is very fortunately situated, so that there 

 is every reason to believe that it will be possible to utilise to 

 the full the resolving power of the instrument. The 100-inch 

 telescope will be admirably adapted to the study of the spectra 

 of faint stars, a direction in which an extension of our know- 

 ledge is badly needed ; to the photography of faint nebulae 

 and other objects of interest ; to follow the variations in 



