•20C) PHILOSOPHICAL TRANSACTIONS. [ANNO 1782. 



tions that fall within the compass of his instruments, and attention to circum- 

 stances necessary to the right management of them, that I have had all along 

 truth and reality in view, as the sole object of my endeavours; and therefore he 

 will be inclined to give some credit to what he does not immediately perceive, 

 when he finds himself successful where he takes the proper precautions so neces- 

 sary in delicate observations, even with the best instruments. I have been in 

 some doubt in what manner to communicate these observations. My first view 

 was to have methodized them properly; but I find them so extensive that there 

 is but little probability that one person should be able to bring them to a con- 

 clusion, for which reason I have now resolved to give them unfinished as they 

 are, that every person who is inclined to engage in this pursuit may become a 

 fellow-labourer. 



In settling the distances of double stars I have occasionally used 2 different 

 ways. Those that are extremely near each other may be estimated by the eye, 

 in measures of their own apparent diameters. For this purpose their distance 

 should not much exceed 2 diameters of the larger, as the eye cannot so well 

 make a good estimation when the interval between them is greater. This 

 method has often the preference to that of the micrometer: for instance, when 

 the diameter of a small star, perhaps not equal to half a second, is double the 

 vacancy between the two stars. Here a micrometer ought to measure lOths of 

 seconds at least, otherwise we could not, with any degree of confidence, rely on 

 its measures; nay, even then, if the stars are situated in the same parallel of 

 declination and near the equator, their quick motion across the micrometer 

 makes it extremely difficult to measure them, and in that case an estimation by 

 the eye is preferable to any other measure ; but this requires not a little practice, 

 precaution, and time, and yet with proper care it will be found that this method 

 is capable of great exactness. Let 2 small circles be drawn, either equal or un- 

 equal, at a distance not exceeding twice the diameter of the larger: let these be 

 shown to several persons in the same light and point of view. Then, if every 

 one of them will separately and carefully write down his estimation of the 

 interval between them, in the proportion of either of their diameters, it will be 

 found on a comparison, that there will seldom be so much as a quarter of a 

 diameter difference among all the estimations. If this agreement takes place 

 with so many different eyes, much more may we expect it in the estimations of 

 the same eye, when accustomed to this kind of judgment. 



I have divided the double stars into several different classes. In the first, I 

 have placed all those which require indeed a very superior telescope, the utmost 

 clearness of air, and every other favourable circumstance to be seen at all, or 

 well enough to judge of them. They seemed to me on that account to deserve a 

 separate place, that an observer might not condemn his instrument or his eye if 

 he should not be successful in distinguishing them. As these are some of the 



