VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 5QI 



for, in the present case, I find by computation, that the intrinsic brightness, so 

 far from being equal in the telescope to that of natural vision, was inferior to it in 

 the ratio of 3 to 7. 



The distinction between magnifying power, and a power of penetrating into 

 space, could not but be felt long ago, though its theory has not been inquired into. 

 This undoubtedly gave rise to the invention of those very useful short telescopes 

 called night-glasses. When the darkness of the evening curtails the natural pene- 

 trating power, they come in very seasonably, to the relief of mariners that are on 

 the look-out for objects which it is their interest to discover. Night-glasses, such 

 as they are now generally made, will have a power of penetrating 6 or 7 times 

 farther into space than the natural eye. For, by the construction of the double 

 eye-glass, these telescopes will magnify 7 or 8 times; and the object-glass being 

 <l\ inches in diameter, the breadth of the optic pencil will be 3-|- or 3-f tenths of 

 an inch. As this cannot enter the eye, on a supposition of an opening of the iris 

 of 1 tenths, we are obliged to increase the value of «, in order to make the tele- 

 scope have its proper effect. Now whether nature will admit of such an enlarge- 

 ment becomes an object of experiment; but, at all events, a cannot be assumed 

 less than-. Then, if x be taken as has been determined for 3 refractions, we shall 



m 



have ^!£2ii^l = 6.46 or 7.39. 



a 



Soon after the discovery of the Georgian planet, a very celebrated observer of 

 the heavens, who has added considerably to our number of telescopic comets and 

 nebulae, expressed his wish, in a letter to me, to know by what method I had been 

 led to suspect this object not to be a star, like others of the same appearance. I 

 have no doubt but that the instrument through which this astronomer generally 

 looked out for comets, had a penetrating power much more than sufficient to show 

 the new planet, since even the natural eye "will reach it. But here we have an 

 instance of the great difference in the effect of the 2 sorts of powers of telescopes; 

 for, on account of the smallness of the planet, a different sort of power, namely, 

 that of magnifying, was required; and, about the time of its discovery, I had been 

 remarkably attentive to an improvement of this power, as I happened to be then much 

 in want of it for my very close double stars.* On examining the nebulae which had 

 been discovered by many celebrated authors, and comparing my observations with 

 the account of them in the Connoissance des Temps for 1783, I found that most 

 of those which I could not resolve into stars with instruments of a small penetrating 

 power, were easily resolved with telescopes of a higher power of this sort ; and that the 

 effect was not owing to the magnifying powerlused on these occasions, will fully ap- 

 pear from the observations; for when the closeness of the stars was such as to require 



* Magnifying powers of 460, 625, 932, 1159, 1504, 2010, 2398, 31 68, 42 4, 54-89, 6450, 

 6652, were used on % Bootis, y Leonis, » Lyrae, &c. See Cat. of double stars, Phil. Trans, vol. 72, 

 page 115, and 147 ; and vol. 75, page 48. — Orig. 



