314 Prof. Amici on the Relative Performances 



an aperture of 36 lines, and I compared with it an achroma- 

 tic telescope of the same length, having an English object- 

 glass of two lenses, 2| inches in diameter. In applying to 

 these instruments two equal eye-glasses, and directing them 

 to the same object, I saw this object with most brightness 

 through the achromatic telescope. In order to be certain of 

 this fact, I constructed a parallelopiped, three inches long, by 

 placing, in opposite directions, two prisms, one of colourless, 

 and the other of obscure glass, such as those which are em- 

 ployed for observing the sun. This apparatus furnished a 

 regular gradation of transparency, by placing it between the 

 eye of the eye-glass, I could easily find the opacity necessary 

 for intercepting entirely the light of the object which I look- 

 ed at alternately with each instrument. In order to avoid all 

 calculation, I diminished by diaphragms the aperture of the 

 brightest telescope^ till the light of the object was extinguish- 

 ed in the two apparatuses at the same division of the parallelo- 

 piped. After several trials I found that, in order that the re- 

 fractor and the reflector should have the same brightness, the 

 first must have an aperture of 27 lines, and the second of 36. 

 I am of opinion, indeed, that this ratio of 3 to 4 ought to be 

 the same for telescopes of all dimensions, as we cannot take 

 into account the slight loss of light arising from the great 

 thickness of the two glasses in these telescopes. 



" In order, then, to see the satellites as bright as I have seen 

 them, we require an achromatic telescope of 8.i inches in dia- 

 meter, and having a focal distance such that the instrument 

 may magnify 400 times. The great object-glass, therefore, of 

 7| inches aperture at Naples, will show the satellites less dis- 

 tinctly than mine, while the 9 inch object-glass at Dorpat will 

 show them more distinctly. 



" The result obtained by Sir W. Herschel does not differ 

 from the above. By the method of Bouguer he found that 

 the diameters of a double achromatic, and of a Newtonian 

 reflector, must be as 7 to 10, in order to produce the same 

 brightness with the same magnifying power, or as 5 to 6, if 

 the small Newtonian mirror is not used. Hence, in order 

 that an achromatic telescope may equal his 40 feet reflector, 

 its object-glass must be 40 English inches in diameter. 





