34 ,6 Dr. Herschel’s Account of the Changes that have happened 
haziness, (which have been related in a paper where the causes 
that often prevent the proper action of mirrors were dis- 
cussed,) show the reason why the apparent distance of a double 
star should be affected by a change in the atmosphere. The 
alteration in the diameter of Arcturus, extending from the first 
to the last of the ten images of that star, in the plate accompany- 
ing the abovemention ed paper,* shows a sufficient cause for an 
increase of the distance of two stars, by a contraction of their 
apparent disks. A skilful observer, however, will soon know 
what state of the air is most proper for estimations of this kind. 
I have occasionally seen the two stars of Castor, from i\ to 2 
and 2-|- diameters asunder; but, in a regular settled temperature 
and clear air, their distance was always the same. The other three 
causes which affect these estimations, are at our own disposal ; 
an instance of this will be seen in the following trial. I took 
ten different mirrors of 7 feet focal length, each having an aper- 
ture of 6,3 inches, and being charged with an eye-glass which 
gave the telescope a magnifying power of 460. With these 
mirrors, one after another, the same evening, I viewed the two 
stars of our double star ; and the result was, that with every 
one of them, the stars were precisely at an equal distance from 
each other. These mirrors were all sufficiently good to show 
minute double stars well ; and such a trial will consequently 
furnish us with a proper criterion, by which we may ascertain the 
goodness of our telescope, and the clearness of the atmosphere 
required for these observations. To those who have not been 
long in the habit of observing double stars, it will be necessary 
to mention, that, when first seen, they will appear nearer toge- 
ther than after a certain time ; nor is it so soon as might be 
* See Phil. Trans, for 1803, page 232, Plate III. 
