1894.] on the Astronomical Telescope. 311 



to transport the instrument to Arequipa or some such favoured 

 locality. No doubt large instruments so placed ought to be, and 

 are, capable of doing much more and better work than if placed in 

 a less favoured spot, say in the neighbourhood of a town, but ex- 

 perience has shown that other influences often arise which militate 

 against the possibility of taking full advantage out of the improved 

 locality. The conditions of life in some such isolated stations are not 

 the pleasantest, and though human nature may put up with incon- 

 veniences and unpleasantness as a temporary arrangement, for the sake 

 of science, yet, as a permanency, this state of things is not found 

 compatible with the production of the best work, and in some cases 

 it has been found necessary to send relays of workers to these 

 isolated stations, a plan, no doubt, which meets to some extent this 

 difficulty, but is evidently open to other objections. 



If however, as it appears, the new photographic system is 

 to a great extent independent of atmospheric disturbance, it ought 

 to be possible not only to use, and use with efficiency, large instru- 

 ments in situations within measurable distance of the haunts of 

 civilisation (a great gain in itself), but it will also be possible to 

 use with advantage, even in such accessible positions, instruments of 

 far greater power than have ever yet been built, and of whose 

 practical value there have been well founded doubts so long as the 

 old system of eye observations was the only one available. 



When this fact forces itself upon the attention of the scientific 

 world, as it must do before long, and the necessity of adding to 

 the power of our telescopes becomes apparent, there is little doubt 

 but that the means will be found to satisfy the necessity ; but as the 

 magnitude of these instruments becomes greater, the importance of 

 studying beforehand the necessary conditions to fulfil and the 

 mistakes to be avoided becomes all the greater, and therefore I have 

 thought it may not be amiss to bring under your notice a few 

 suggestions as to the possibility of obtaining increased optical power 

 in our telescopes. 



Before we discuss the conditions to be fulfilled in the case of the 

 mounting of more powerful telescopes, perhaps it would be well to 

 get a clear idea of what is meant by the power of a telescope as 

 distinct from magnifying power. You are aware that most of the 

 work done with our very large refractors is done with magnifying 

 powers which are equally useable with instruments of half the aper- 

 ture or less, but it must not be assumed that the power of the 

 instrument (used in its broad sense, i.e. its capability of distinctly 

 viewing minute objects, or the details of such) is then only the same 

 as that of the smaller instrument used with the same magnifying 

 power. On the contrary, Jupiter or Saturn viewed with a power of 

 600, with 23 inches aperture, is a very different object to what it is 

 when viewed with the same magnifying power and an aperture of say 

 8 or 10 inches. This is not due to extra brilliancy from the larger 

 amount of light collected by the bigger object-glass, for even in the 



