196 



The Rev. Dr. Robinson on the Constant of Refraction. 



and its reflected image) does not take place without a minute depression of the 

 mercury, which is between 0.001 and 0.002. 



The refractions have been computed with ji — 57.7682 (Brinkley's reduced 

 to my latitude), and the colatitude 35° 38' 47". 3. In this climate and this 

 exposed situation, it is not very easy to observe by reflection, and I have not yet 

 definitively settled this element. 



With the first division of the circle, 41 pair give 47". 22 

 With the second „ 58 „ 47". 48 



With the third „ 132 „ 47". 37 



mean . 47". 37 

 The first and third are corrected for error of division. In the second, three 

 divisions were read at each microscope. It is obvious that these give no reason 

 for changing 47".3, which had previously been determined with Troughton's 

 equatorial by upwards of 200 pair of observations ; and equally so that whatever 

 uncertainty there be, can have no effect. 



The following are the results that I have obtained : 



45 up' Cygni. 



Twelve observations (1838. 772) with Brinkley's Constant of Refraction 

 give the Declination for 1830, 



8= + 48° 23' 1". 51. 



Precession = + 11". 844 ; sec var. = -f 0". ^12 ; proper motion = + 0".033. 



* The figures after the minutes of zenith distance are decimals. 



