of the atmosphere, &c. 381 
But Dr. Bradley undoubtedly made use of dark glasses 
For observing the sun, probably smoked glasses, which would 
give him a pale orange coloured image, or one of less than 
mean refrangibility ; 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 Almanac 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 refrangibility than smoked glass. The effect 
of this alteration, therefore, should have been, that arising 
from too great correction for refraction in every thing de- 
pending on observations of the sun. 
The introduction of achromatic object glasses * produced an 
error of a different kind ; and one which, in certain cases, 
tends to correct the other. In the single object glass tele- 
scope (and there were no others in Bradley’s time) the dif- 
ferently coloured images are formed at different focal dis- 
tances, which, in a manner, compels the observer to adjust 
his instrument to the most intense light ; that is to say, to the 
orange coloured -f image ; by this means the fainter colours, 
* An achromatic object glass was first applied to the south quadrant at Greenwich 
in 1772, and to the north quadrant in 1789. 
f Vide Newton’s Optics, Book I. Part I. Prop. VII* 
