102 
Mr, herschel on the 
between them, as in fig. 6. which represents a Geminorum as I 
have often feen it with a power of 460. It will be neceffary 
here to take notice, that the eftimations made with one tele- 
fcope cannot be applied to thofe made with another : nor can 
the eftimations made with different powers, thougn with the 
fame telefcope, be applied to each other. Whatever may be the 
caule of the apparent diameters of the flars, thej' ate certainly 
not of equal magnitude with the fame powers in different tele- 
Scopes, nor of proportional magnitude with different powcis 111 
the fame telefcope. In my inffruments I have ever found lefs 
diameter in proportion the higher I was able to go in power, 
and never have I found fo Small a proportional diameter as 
when I magnified 6450 times*; therefore if we would wifh 
to compare any fuch obfervations together, with a view to fee 
whether a change in the diflance has taken place, it fhould be 
done with the very fame telefcope and power, even with the 
very fame eye-glafs or glaffes ; for others, though of equal 
power and goodnefs, would moft probably give different pro- 
portional diameters of the liars. 
In the third clafs I have placed all thofe double flars that 
are more than five but lefs than 1 5" afunder ; and for that rea- 
fon, if they fhould be ufed for obfervations on the parallax of 
the fixed flars, they ought not to be looked upon as quite free 
from the effeds of refradion, &c. In the fame manner that 
the flars in the firff and Second claffes will ferve to try the 
goodnefs of the moil capital inffruments, thefe will afford ob- 
jeds for telefcopes of inferior power, fuch as magnify from 40 
to 100 times. The obferver may take them in this or the like 
order: Urfe majoris, y Delphini, y Arietis, 7r Bootis, y Vir- 
* See the meaiures of ihc diameter of « Lyrce, Catalogue of double flars, 
5th clafs. . . 
gnus. 
