1 24 On the Size lest adapted for Achromatic Glasses. 



iiess of the pencil of rays, which will of course be in proportiorr 

 to the brilliance of the object observed. 



Some stars I have observed with a power which diminished 

 the diameter of the pencil to nearly one hundred-and-twentieth 

 of an inch ; i. e. a power of 420, with an aperture of three inches 

 and five-eighths diameter in the clear. I have never yet seen 

 any object that appeared to recpiire a greater power; and it re- 

 qTiires a most perfect telescope, and every other favourable cir- 

 cumstafnce, to admit of this being used with any advantage. From 

 the rapidity of the diurnal motion of the earth, the limited ex- 

 citability of the eye, the impediments to vision arising from our 

 magnifying the atmospheric medium we look through, in propor- 

 tion as we magnify the object we look at, increase in so high a 

 ratio to the magnifying power, that more than 100 for terrestrial 

 and 300 for astronomical use rather impede than assist vision. 

 And again, when we charge our telescopes with a higlier power 

 than 300 times, what very unconunon dexterity is required either 

 to find the object, or manage the instrument ! It is indeed for- 

 timate that a higher magnifier is rarely needful, as it cannot be 

 used to much advantage till the atmosphere be removed, and the 

 earth stands till. 



With a good achromatic of forty-six inch focus, and a treble 

 object-glass of three inches and five-eighths in the clear aper- 

 ture, I have seen that most minute point of light near the pole- 

 star, with tlie following powers, measured by a dynameter in- 

 vented and made by the late ingenious Mr. Ramsden : 40, SO, 

 150, 250, 350, 420, 700; and "even with 1123 times the small 

 star was still visible. Mr. William Walker, the astronomer, was 

 observing with me, and also saw this. Mr. Charles Fairbone, 

 mathematical-instrument maker, of Great New-Street, Fetter- 

 Lane, saw it again very distinctly on the 30th August 1807. 

 Mr. Samuel Pierce, telescope-maker, at Mr. Berge's, optician, 

 Piccadilly, observed the same on the 26th PJay 1811. 1 believe 

 the polar star is as proper as any, for a test of the perfection of 

 a telescope, as to its light and distinctness ; and as it is easily 

 found, and always visible, it is the more desirable, as it is a more 

 universally attainable test. 



I mention the foregoing observations merely as an authenti- 

 cated and curious fact, how far magnifying power could be car- 

 ried on this object, as it was with evident detriment to vision 

 when higher than 80, which showed this star more pleasantly, 

 and the illuminating and magnifying j^owers for this object ap- 

 peared to be in more perfect proportion than with any of the 

 higher or the lower powers. 



We should never use a greater magnifier than we absolutely 

 want J the lower the power, the more beautiful and brilliant the 



pbject 



