in the relative Situation of double Stars, &c, 345 
a Geminorum. 
From my earliest observations on the distance of the two stars 
which make up the double star in the head of Castor, given in 
the first of my catalogues of double stars, we find, that about 23 
years and a half ago, they were nearly two diameters of the 
large star asunder. These observations have been regularly con- 
tinued, from the year 1778 to the present time, and no alteration 
in the distance has been perceived : the stars are now still nearly 
2 diameters of the large one asunder. 
It will be necessary to enter a little into the practicability of 
ascertaining distances by a method of estimation apparently so 
little capable of precision. From a number. of observations and 
experiments I have made on the subject, it is certain that the 
apparent diameter of a star, in a reflecting telescope, depends 
chiefly upon the four following circumstances : the aperture of 
the mirror with respect to its focal length ; the distinctness of the 
mirror ; the magnifying power ; and the state of the atmosphere 
at the time of observation. By a contraction of the aperture, 
we can increase the apparent diameter of a star, so as to make 
it resemble a small planetary disk. If distinctness should be 
wanting, it is evident that the image of objects will not be sharp 
and well defined, and that they will consequently appear larger 
than they ought. The effect of magnifying power is, to occasion 
a relative increase of the vacancy between two stars that are 
very near each other ; but the ratio of the increase of the dis- 
tance is not proportional to that of the power, and sooner or 
later comes to a maximum. The state of the atmosphere is per- 
haps the most material of the four conditions, as we have it not 
in our power to alter it. The effects of moisture, damp air, and 
