GREAT REFRACTING TELESCOPE AT ELCHIES, IN MORAYSHIRE. 415 



The next question will be, as to the defining power of the telescope. 



This I believe to be good, though the atmosphere was almost invariably so 

 disturbed, that I never could try any very severe test. On one, however, of the 

 only two occasions on which there was good atmospheric definition, I was look- 

 ing at 7 Andromedse, and had most admirable proof that the smallness of the discs 

 of the stars was just as it should be, with an excellent quality of object-glass of 

 the given size; for, while the Cycle 5-9-inch just barely notched the oblong spot 

 of light formed by the two close stars B and C; and while the Pattinson 7'25-inch 

 on Teneriffe showed them separate, with a black line between them, but yet with 

 their discs mutually compressed on the adjacent sides, and without which com- 

 pression they must have touched, — the Elchies 11-inch showed them completely 

 separated, and perfectly round ; the distance apart of the two stars being about 

 0"*6, and being believed to have been constant through the three observations. 



To these optical advantages, depending on size accompanied by excellence, 

 the Elchies telescope also realized, and showed that there is great satisfaction to 

 the observer in fully experiencing the mechanical advantages of greater steadi- 

 ness of motion, freedom from tremors, or disturbance by wind, and constancy'' 

 of zero points,* as necessarily imparted by a large and heavy instrument, than 

 trifled with by all the smaller and lighter ones ever used hy the same ohserver. 



Part C. — (2.) Gosmical Results. 



The series of these Elchies observations is too small to expect much from them 

 of this order in the present very advanced condition of sidereal astronomy ; but 

 still I do trust that faithful labour with so powerful an instrument has not gone 

 altogether unrewarded. 



Beginning with the first of our four telescopic observed quantities, viz., Mag- 



* The instances in Correction-Table II., p. 377, having been further examined for the often 

 reputed effect of absolute position angle in biasing more or less the estimation of any observer in 

 both position and distance, have given some indications of a small correction of that nature, varying 

 with the position, being required as below ; the distances having been first corrected for a probable 

 residual error, both of run of micrometer and method of measuring;, amounting at a distance of 5" to 

 + -04", at 10" to -f-'Ol", at 15" to --03", at 20" to --08", and a^t 25" to --13". 



Position 



Correction in 



Position 



Correction in 



Position 



Correction in 



Angle. 



Position. 



Distance. 



Angle. 



Position. 



Distance. 



Angle. 



Position. 



Distance. 



o 







-13 



+ •07 



120 



+ 4 



-■01 



240 



6 



-•03 



20 



- 9 



+ •07 



140 



+ 4 



-•02 



260 



- 3 



-•02 



40 



- 4 



+ •06 



160 



+ 3 



-•03 



280 



- 6 



•00 



60 







+ •04 



180 



+ 3 



-•04 



300 



-10 



+ •02 



80 



+ 2 



+ •03 



200 



+ 2 



-•04 



320 



-13 



+ •04 



100 



+ 3 



+ •01 



220 



+ 1 



-•03 



340 



-15 



+ •06 



120 



+ 4 



- 01 



240 







-•03 



360 



-13 



+ •07 



These corrections, though regular, are so small, compared to the ordinary error of individual 

 observations, that they have not yet been applied in any case. 



VOL. XXIII. PAKT 11. ' 5 U 



