208 PHILOSOPHICAL TRANSACTIONS. [ANNO 1782. 



star, perfectly round and well defined, with a deep black division between them, 

 as in fig. 6, which represents a Geminorum as I have often seen it with a power 

 of 46o. It will be necessary here to take notice, that the estimations made with 

 one telescope cannot be applied to those made with another: nor can the estima- 

 tions made with different powers, though with the same telescope, be applied to 

 each other. Whatever may be the cause of the apparent diameters of the stars, 

 they are certainly not of equal magnitude with the same powers in different tele- 

 scopes, nor of proportional magnitude with different powers in the same tele- 

 scope. In my instruments I have ever found less diameter in proportion the 

 higher I was able to go in power, and never have I found so small a proportional 

 diameter as when I magnified 6450 times;* therefore if we would wish to com- 

 pare any such observations together, with a view to see whether a change in the 

 distance has taken place, it should be done with the very same telescope and 

 power, even with the very same eye-glass or glasses; for others, though of equal 

 power and goodness, would most probably give different proportional diameters 

 of the stars. 



In the 3d class I have placed all those double stars that are more than 5 but 

 less than 15" asunder; and for that reason, if they should be used for observa- 

 tions on the parallax of the fixed stars, they ought to be considered as quite free 

 from the effects of refraction, &c. In the same manner that the stars in the 1st 

 and 2d classes will serve to try the goodness of the most capital instruments, 

 these will afford objects for telescopes of inferior power, such as magnify from 

 40 to 100 times. The observer may take them in this or the like order: £ Ursas 

 majoris, y Delphini, y Arietis, -w Bootis, y Virginis, i Cassiopeae, ^ Cygni. 

 And if he can see all these, he may pass over into the 2d class, and direct his 

 instrument to some of those that were pointed out as objects for the very best 

 telescopes, where I suppose he will soon find the want of superior power. 



The 4th, 5th, and 6th classes contain double stars that are from 15 to 30'', 

 from 30" to l', and from 1' to l' or more asunder. Though these will hardly 

 be of any service for the purpose of parallax, I thought it not amiss to give an 

 account of such as I have observed; they may perhaps answer another very im- 

 portant end, which also requires a great deal of accuracy, though not quite so 

 much as the investigation of the parallax of the fixed stars. I will just men- 

 tion it, though foreign to my present purpose. Several stars of the first mag- 

 nitude have already been observed, and others suspected, to have a proper mo- 

 tion of their own; hence we may surmise, that our sun, with all its planets and 

 comets, may also have a motion towards some particular part of the heavens, 

 on account of a greater quantity of matter collected in a number of stars and 



* See the measures of the diameter of » Lyrce. Catalogue of double stars, 6th class. — Orig. 



