YOL. LXXII.] PHILOSOPHICAL TRANSACTIONS. 207 



finest, most minute, and most delicate objects of vision I ever beheld, I shall be 

 happy to hear that my observations have been verified by other persons, which 

 I make no doubt the curious in astronomy will soon undertake. I should 

 observe, that since it will require no common stretch of power and distinctness 

 to see these double stars, it will therefore not be amiss to go gradually through a 

 few preparatory steps of vision, such as the following: when n Coronas borealis, 

 one of the most minute double stars, is proposed to be viewed, let the telescope 

 be some time before directed to a Geminorum, or if not in view to either of 

 the following stars, £ Aquarii, y. Draconis, g Herculis, a Piscium, or the curious 

 double-double star £ Lyrae. These should be kept in view for a considerable 

 time, that the eye may acquire the habit of seeing such objects well and dis- 

 tinctly. The observer may next proceed to £ Ursas majoris, and the beautiful 

 treble star in Monoceros's right fore-foot; after these to i Bootis, which is a fine 

 miniature of x Geminorum, to the star preceding a. Orionis, and to n Orionis. 

 By this time both the eye and the telescope will be prepared for a still finer 

 picture, which is n Coronas borealis. It will be in vain to attempt this latter if all the 

 former, at least i Bootis, cannot be distinctly perceived to be fairly separated, 

 because it "is almost as fine a miniature of i Bootis as that is of a. Geminorum. 

 If the observer has been successful in all these, he may then, at the same time, 

 try h Draconis, though I question whether any power less than 4 or 500 will 

 show it to be double; but all the former I have seen very well with 227. 



To try the stars of unequal magnitudes it will be expedient to take them in 

 some such order as the following: a Herculis, u Aurigae, S Geminorum, k 

 Cygni, e Persei, and b Draconis; from these the observer may proeeed to a most 

 beautiful object, £ Bootis which I have closely attended these 2 years as very 

 proper for the investigation of the parallax of the fixed stars. 



It appears, from what has been said, that these double stars are a most excel- 

 lent way of trying a telescope; and as the foregoing remarks have suggested the 

 method of seeing how far the power and distinctness of our instruments will 

 reach, I shall add the way of finding how much light we have. The observer 

 may begin with the pole star and » Lyras ; then go to the star south of i Aquilas, 

 the treble star near h Aquilae, and last of all to the star following o Aquilae. 

 Now, if his telescope has not a great deal of good distinct light, be will not be 

 able to see some of the small stars that accompany them. 



In the 2d class of double stars, I have put all those that are proper for esti- 

 mations by the eye or very delicate measures of the micrometer. To compare 

 the distances with the apparent diameters, the power of the telescope should not 

 be much less than 200, as they will otherwise be too close for the purpose. The 

 instrument ought also to be as much as possible free from rays that surround a 

 star in common telescopes, and should give the apparent diameters of a double 



