J5» Penetration into Space by Tele/copes. 



The diftinflion between magnifying power, and a power of penetrating into fpace, 

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

 doubtedly gave rife to the invention of thofe very ufeful fliort telefcopes called night- 

 glafles. When the darknefs of the evening curtails the natural penetrating power, they 

 come in very feafonably, to the relief of mariners that are on the look-out for obje£ls 

 which it is their intereft to difcover. Night-glafles, fuch as they are now generally made, 

 will have a power of penetrating fix or feven times farther into fpace than the natural eye. 

 For, by the conftruftion of the double eye-glafs, thefe telefcopes will magnify 7 or 8 

 times i and the obje£l glafs being 2i inches in diameter, the breadth of the optic pencil 

 will be 3 1 or 3^ tenths of an inch. As this cannot enter the. eye, on a fuppofition of an 

 opening of the iris of 2 tenths, we are obliged to increafe the value of a, in order to make 

 the telefcope have its proper effect. Now, whether nature will admit of fuch an enlarge- 

 ment becomes an objc£t of experiment ; but, at all events, a cannot be aflumed lefs than 



. Then, if x be taken as has been determined for three refractions, we (hall have 



^/,853 X 25 



^ = 6,46 or 7,39. 



Soon after the difcovery of the Georgian planet, a very celebrated obferver of the 

 heavens, who has added confiderably to our number of telefcopic comets and nebulae, ex- 

 prefled his wifli, in a letter to me, to know by what method I had been led to fufpe£l this 

 obje£l not to be a ftar, like others of the fame appearance. I have no doubt but that the 

 inftrument through which this aftronomer generally looked out for comets, had a pene- 

 trating power much more than fuflicient to ftiew the new planet, fince even the natural eye 

 will reach it. But here we have an inftance of the great difference in the effeft of the two 

 forts of powers of telefcopes ; for, on account of the fmallnefs of the planet, a different 

 fort of power, namely, that of magnifying, was required ; and, about the time of its 

 difcovery, I had been remarkably attentive to an improvement of this power, as I hap- 

 pened to be then much in want of it for my very clofe double ftars *. 



On examining the nebulae which had been difcovered by many celebrated authors, and 

 comparing my obfervations with the account of them in the Connoijfance des Temps for 

 1783, I found that mod of thofe which I could not refolve into ftars with inllruments of a 

 fmall penetrating power, were eafily refolved with telefcopes of a higher power of this fort ; 

 and, that the effedl was not owing to the magnifying power I ufed upon thefe occaGons, 

 will fully appear from the obfervations : for, when the clofenefs of the ftars was fuch as to 

 require a confiderable degree of magnifying as well as penetrating power, it always ap- 

 peared plainly, that the inftrument which had the highcft penetrating power refolved them 

 beft, provided it had as much of the other power as was required for the purpofe. 



Magnifying powers of 460, 615,931, 1159, 150*, 2010, 4398, 316S, 4194, 5+?9, 6450,6652, were 

 ofed upon t Bootis, 7 Leonis, a. Lyrse, t^c. See Cat. of .double ftars, Phil. Tranf. Vol. LXXII. page 115, 

 an<i'i47i andVol.LXXV, page 48. 



3 Sept. 



