40 On the dispersive Power of the Amosphere, €c. 
him a pale orange-coloured image, or one of less than mean re- 
frangibility; consequently, the quantity of refraction as found 
by Dr. Bradley must be too small for white light. 
This alone is sufficient to produce a small difference between 
the results of our observations of the sun and of the stars. I 
shall now mention two other circumstances which appear to me 
to have produced a still greater apparent disagreement. 
The publication of the Nautical Almanack in 1767, led to the 
general use of Hadley’s sextant. In the construction of this 
instrument, coloured glasses were indispensibly necessary; and 
the great convenience in the use of them over smoked glasses, 
soon occasioned the application of them to all other instruments. 
These glasses generally give a deep-red image, or one of less re- 
frangibility than smoked glass. The effect of this alteration, 
therefore, should have been, that arising from too great correc- 
tion for refraction in every thing depending on observations of 
the sun. 
The introduction of achromatic cbject-glasses* produced an 
error of a different kind; and one which, in certain cases, tends 
to correct the other. In the single object-glass telescope (and 
there were no others in Bradley’s time) the differently coloured 
images are formed at different focal distances, which, in a man- 
ner, compels the observer to adjust his instrument to the most 
intense light; that is to say, to the orange-coloured + image ; by 
this means the fainter colours, which occupy the greatest space 
in the spectrum ¢, are dissipated, and lost among the more power- 
ful rays. In good achromatic telescopes the case is very dif- 
ferent, 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 extremity of the green image. 
But if the upper extremity of the green image be taken in’ 
observations of cireumpolar stars, a greater correction than Dr. 
Bradiey’s ought to be applied, in order to get the true height of 
the pole. 
It may not be amiss to cbserve 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, 
Greenwich in 1772, and to the north quadrant in 1789, 
t Vide Newton’s Optics, book [. part i. prop. Vil, 
} Ibid. book J, part 11. prop, iii, 
* An achromatic object-glass was first applied to the south quadrant at 
being 
