On the Size lest adapted for Achromatic Glasses. 123 



seven feet Newtonian has sufficient light with a single eye-glass, 

 which gives it a magnifying power of 287, to show the belts and 

 double ring of Saturn completely well. What can we wish for 

 Tiiore ? How many have expended large sums of money on te- 

 lescopes, without having ever seen such an all-repaying sight ! 



Query: Can the acme of perfection be obtained in metals of 

 larger diameter ? Several of our first-rate practical and working 

 opticians have candidly declared to me, they would not, for ge- 

 neral sale, undertake to make speculums of larger size than nine 

 inches, that would show a star round and neatly: and unless thev 

 will bear this grand ordeal, it has been the fasliion, lately, to 

 suppose its figure cannot be depended on for exhibiting any ob- 

 ject with that faithful accuracy which is the sine qua noa of 

 astronomical instruments. 



That distinctness of vision appearing to""be so limited, may 

 not create one sigh from the breast of any minute philosopher, 

 that further optical assistance cannot be given to his eye; and 

 that art is, as 1 have before said, so circumscribed ; I will ven- 

 ture to account for these impediments and boundaries from the 

 operations of Nature herself; i.e. the rapid rotatory motion of 

 the earth preventing the application of a higher power than 300 

 times being used with any advantage. This is so true, that, un- 

 til this obstacle is removed, we need not hunt after monstrous 

 telescopes, unless it be in the true hobby-horsical spirit, /br the 

 sake of the pleasure arising from the trouble ofming them, and 

 being disappointed. Beyond a certain size, telescopes are only 

 just as useful, as the enormous spectacles which are suspended 

 over the doors of opticians. 



When the inventors of the achromatic glasses fixed the po^vers 

 of their telescopes, it was no doubt done after due deliberation, 

 and a conviction arising from experiment, that for planetary uses 

 the proportion of the pencil of rays to the diameter of the ob- 

 ject-glass was most proper when as forty to one, /. e. for common 

 telescopes and common observers. Thus the thirty-inch, with 

 two inches aperture, magnifies eighty times; and it may be con- 

 sidered a general rule, that to find the most effective magnifving 

 power of a telescope for planetary use, multiply the diameter of 

 the object-glass by forty or fifty: to bear more it must be a very 

 fine instrument, and the planet near the meridian ; by the proxi- 

 mity of the object to which, the application of magnifying power 

 must always be governed. When the jiencil is much less than 

 one-fiftieth of an inch diameter, it is too diluted to perfectly 

 excite the action of the eye : and, when applied to the planets, 

 we lose in distinctness more than we gain, by the magnifying 

 being in too high a ratio to the illuminating power. But we 

 must take into the account not only the bigness but the bright- 

 ness 



