294 Mr. Pontl on the Parallax of u L^ne. 



periment is tried with two microscopes only ; for to suppose 

 the contrary, would be to deny the tendency of six micro- 

 scopes to correct the errors of two. Now I find the same 

 difference of polar distance whether I employ two microscopes 

 or six; temperature, therefore, cannot materially have viti- 

 ated the results by causing derangement in the form of the 

 instrument. 



In the whole of the above process I do not see one objec- 

 tionable point, and if called upon to invent an instrument for 

 this particular experiment, I could not devise one more per- 

 fect in principle than the mural circle. 



Whoever will compare the above simple process with the 

 more comphcated one necessarily employed in using an in- 

 strument with two microscopes, turning freely in azimuth, 

 will not hesitate, I think, in deciding upon which of the two 

 instruments tempeniture is likely to produce the greatest error. 

 On the absolute Parallax of a. Lyrce. 



The preceding observations only indicate that y Draconis 

 and a Lyrae have the same parallax, or that their difference 

 of parallax is zero ; but they have no tendency to show what 

 is the actual magnitude of the parallax that the two stars have 

 in common. If indeed we admit it to be proved, by the ob- 

 servations of Bradley, and the more recent ones of Dr. Brinkley, 

 that the parallax of y Draconis is insensible, we may then infer 

 from the observed difference what is the parallax of the other 

 .star. But the method of investigation that we are now about 

 to consider, does not depend on such an admission. 



Having successfully adopted the method of observing by 

 reflection, I was desirous of employing it in a series of obser- 

 vations upon « Lyrae, with a view to determine this question. 

 This series began on the 1st of July 1822, and has been con- 

 tinued to the present time *. Although this period embraces 

 only half the interval in which the greatest change or double 

 parallax is affected, a circumstance which at first may appear 

 very disadvantageous, yet that is more than compensated, in 

 my opinion, by the number of observations, and by a unifor- 

 mity of temperature, such as never can be expected in the 

 extreme seasons of whiter and summer. 



In observations of this nature the effects of temperature 

 ujion the instrument itself, and the uncertain refractions of 

 tlie ray of light when brought into the lower part of the room, 

 may produce errors of no inconsiderable magnitude, with re- 

 ference to a questioji of so much nicety as the present. 



* Since the date of this paper (read Nov. 1 4, 1 822) the observations have 

 been continued throughout the winter, and the results will be found in the 

 Table, Phil. Trans, p. 61. 



I can 



