382 Mr. Lee on the dispersive power 
which occupy the greatest space in the spectrum,* are dissi- 
pated, and lost among the more powerful rays. In good 
achromatic telescopes the case is very different, for all the 
rays being collected by them into one point, every colour is 
seen in its proper place, so that the observer, in bisecting the 
spectrum, takes the altitude of the mean, or the upper ex- 
tremity of the green image. 
But if the upper extremity of the green image be taken in 
observations of circumpolar stars, a greater correction than 
Dr. Bradley's ought to be applied, in order to get the true 
height of the pole. 
It may not be amiss to observe here, that the observations 
of Mr. Lalande at Paris, show a greater disagreement than 
those at Greenwich ; and the observations of Mr. Piazzi at 
Palermo, a still greater than those of Mr. Lalande. This, 
I apprehend, must arise partly from the lesser elevation of 
the pole in those places, and partly from the fainter colours 
in the stellar spectra being more distinctly visible in the clear 
atmospheres of France and Italy than in England. 
It should seem then, that in order to get a perfect know- 
ledge of astronomical refraction, we ought to employ at least 
three different methods of investigation. 1st. By observations 
of the fixed stars during the night, when all the prismatic 
colours are visible. 2dly. By observations of the stars during 
the day, when none but the orange coloured rays are to be seen. 
And 3rdly, by observations of the sun with different coloured 
glasses. By these means we might hope to obtain such an 
accurate knowledge of atmospheric refraction as would enable 
us to form tables adapted to every possible circumstance. 
• Vide Newton’s Optics, Book I. Part II. Prop. IIL 
