of the Cassegrainian Telescope ^ C^c. 209 



From this experiment it appears, that the light in both te- 

 lescopes was equal when the area of the aperture of the Cas- 

 segrainian, was to that of the Gregorian, as 4,632 to 10,871. 

 Now the increase of light being (under similar circumstances) 

 directly as the area of the aperture, it follows that if the aper- 

 ture of the Cassegrainian be made equal to that of the Grego- 

 rian, the light in favour of the former will be as 10,871 to 

 4,63a, or in the surprising proportion of 7 to 3 nearly. 



A difference of such magnitude could not be admitted but 

 with extreme caution, particularly as the Gregorian telescope 

 had been made some time, and its mirrors might therefore be 

 supposed not to possess so high a polish, as those of the Cas- 

 segrainian which had been recently finished ; but I was soon 

 enabled to pursue the subject, as a Gregorian telescope was 

 made by Mr. Ck?ckmore fully equal, if not superior, to any 

 he had before constructed ; the mirrors were of an exquisite 

 polish. The Cassegrainian, used in this experiment, was the 

 one I formerly mentioned, the aperture of which was five 

 inches, and the length thirty inches. It had not been carefully 

 preserved, and the large mirror had lost somewhat of its ori- 

 ginal polish. All circumstances being thus in favour of the 

 Gregorian, a paste-board circle was prepared, and the expe- 

 rimei.t conducted as before. When the images of the card 

 were ecfually bright, the following measures were taken. 



