VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 5Q7 



nifying may possibly not exceed the reach of a 20 or 25-feet telescope ; or may 

 even lie in a less compass than either. However, in beautiful nights, when the 

 outside of our telescope is dropping with moisture discharged from the atmosphere, 

 there are now and then favourable hours, in which it is hardly possible to put a 

 limit to magnifying power. But such valuable opportunites are extremely 

 rare ; and, with large instruments, it will always be lost labour to observe at other 

 times. 



As I have hinted at the natural limits of magnifying power, I shall venture also 

 to extend my surmises to those of penetrating power. There seems to be room for 

 a considerable increase in this branch of the telescope ; and, as the penetrating 

 power of my 40- feet reflector already goes to 191.69, there can hardly be any 

 doubt but that it might be carried to 500, and probably not much farther. The 

 natural limit seems to be an equation between the faintest star that can be made 

 visible, by any means, and the united brilliancy of star-light. For, as the light of 

 the heavens, in clear nights, is already very considerable in my large telescope, it 

 must in the end be so increased, by enlarging the penetrating power, as to become 

 a balance to the light of all objects that are so remote as not to exceed in brightness 

 the general light of the heavens. Now if p be put for penetrating power, we have 

 /^- = a = 10 feet 5.2 inches, for an aperture of a reflector, on my construc- 

 tion, that would have such a power of 500. 



But, to return to our subject ; from what has been said before, we may conclude, 

 that objects are viewed in their greatest perfection when, in penetrating space, the 

 magnifying power is so low as only to be sufficient to show the object well ; and 

 when, in magnifying objects, by way of examining them minutely, the space-pene- 

 trating power is no higher than what will suffice for the purpose ; for, in the use of 

 either power, the injudicious overcharge of the other, will prove hurtful to perfect 

 vision. 



It is remarkable that, from very different principles, I have formerly determined 

 the length of the visual ray of my 20-feet telescope on the stars of the milky way, 

 so as to agree nearly with the calculations that have been given.* The extent of 

 what I then figuratively called my sounding line, and what now appears to answer 

 to the power of penetrating into space, was shown to be not less than 415, 46 1, 

 and 497 times the distance of Sirius from the sun. We now have calculated that 

 my telescope, in the Newtonian form, at the time when the paper on the Con- 

 struction of the Heavens was written, possessed a power of penetration, which ex- 

 ceeded that of natural vision 6 1.1 8 times; and, as we have also shown, that stars 

 at 8, 9, or at most 10 times the distance of Sirius, must become invisible to the eye, 

 we may safely conclude, that no single star above 489, 551, or at most 612 times 

 as far as Sirius, can any longer be seen in this telescope. Now the greatest length 

 of the former visual ray, 497, agrees nearly with the lowest of these present num- 

 bers, 489 5 an d tne higher ones are all in favour of the former computation ; for 



* Phil. Trans, vol. 75, p. 217, 248.— Orig. 



