M. Bessel on the Declination ofthejixed Stars. 27 



account I have sought and employed them by the following 

 results. 



In the introduction to the 7th Number of my Observations, 

 which is now in the press, I have described the whole of my 

 process. In this place I shall merely touch upon such par- 

 ticulars as appear to me indispensable. The errors of divi- 

 sion I have determined by a very beautiful apparatus of four 

 microscopes prepared by INIr. Pri\y Counsellor Pistor ; from 

 whence it appears that these errors are very small, but still 

 not quite irregular. These UTegularities amount for each line 

 probably to +0",3251 only; whence it follows that probably 

 about one only in twenty-six of the lines varies 1" or more, 

 and that among the whole 7200 lines on the circle only two 

 vary between 1",75 and 2". Through these slight irregularities, 

 regular errors in the divisions are recognised, whose maxi- 

 mum never reaches 1,"25; but which through the use of the 

 four verniers are mutually done away with, so that the pre- 

 ceding correction does not exceed 0",33. This almost inci'e- 

 dible perfection of division in a circle of 1 8 inches radius, can- 

 not but excite the admiration of every one. I find the bending of 

 the telescope in the horizon =1",11, whilst it is not quite =0 

 in the zenith, according to my researches. The refraction T 

 have determined by very numerous observations from 59 cir- 

 cum-]iolar stars, and these were found to vaiy but little from 

 the table \ni\\QFundamentaAstronomice,\\7.. for the tempei'ature 

 of 48°,75 in the proportion of 1 to 1,003282 greater, which 

 difference would even almost entirely disappear, if I had 

 omitted the suggested correction of — 1°,25 in Bradley's ther- 

 mometer, as well as the remark of Professor Tralles, that the 

 thermometer appears to give the freezing point too high, when 

 immersed in melted snow mixed with water. I have more- 

 over considered the influence of the state of the thermometer 

 on the refraction in the course of my observations, and found 

 it about a 35th smaller, than it would be by the proportion of 

 1 to 1,375 : for the reasons which induced me to suspect that 

 these would be smaller I must refer to the 7th Number of my 

 Observations already mentioned. 



The result of these examinations has been, that the declina- 

 tions of the stars from the pole to « Lyrce, wliether the obser- 

 vations were on the east or west side of the circle, or derived 

 from the upper or lower culmination, agree as nearly as the 

 small incidentjil errors of obsei'vation would lead one to ex- 

 pect. The same agreement on bodi sides of the instrument 

 IS likewise sliown by the fundamental stars culminating south 

 of the zenith. 



I bore give two tables of the declination of these stars. Tlui 

 I) 2 first 



