2] THE SEARCH FOR THE PLANET NEPTUNE. 45 



My observations were all made with the large Northumberland "Refractor, 

 and with a magnifying power of 170. On August 4, the Hour Circle being 

 fixed, the telescope was moved in declination, and the transits were all 

 taken at the same part of the field, at the toothed edge of the comb of 

 a micrometer eye-piece. Differences of declination were measured by means 

 of a graduated sector-arc, which was read off by a microscope-micrometer, 

 one revolution of which is 10". The stars were accurately bisected by a 

 fixed wire equator! ally adjusted, but to gain time the micrometer was read 

 off to integral revolutions, and by estimation to a fourth part of a revo- 

 lution. The error of reading off in this way could hardly be more than 

 3", and the error of comparison with a single star might possibly amount 

 to 6". On August 12, the telescope was absolutely fixed, and the zone, 

 which was 9' in breadth, was limited by the field of view. The transits 

 were taken at the toothed edge of the comb carefully adjusted, and the 

 differences of declination were measured by revolutions of the eye-piece 

 micrometer, read off in integral revolutions, and by estimation to a fourth 

 part of a revolution, by means of the teeth of the comb. Occasionally, as 

 it happened in the instance of the planet, the tenth part of a revolution 

 was estimated. The value of one revolution of this micrometer is 17", and 

 I should therefore estimate the error of comparison with a single star, so 

 far as it depended on error of reading off, to be at most 8". I now give 

 the places of the planet resulting from a comparison with every known 

 star that was taken in the same series on each of the two days. 



August 4 



Star of Comparison and authority . h f pj d f p , t 



for its place. 



. . . 01 <> 



. fBritish Association Catalogue ... 2 1 58 14' 13 ...... - 12 57 18'4l 



g 



h. m. s. 



. 



50 Capncorm ...... ...... 



British Association Catalogue 7599 ............ 14'89 ...... 32'0 



38 Aquarii B. A. C. 7722 h . m. s. ...... 14-86 ...... 41'9* 



Bessel Z. 127 and Z. 129 215910 ...... 14-48 ...... 33-6 



127 and Z. 129 22 5 6 ...... 14'80 ...... 34'9 



-- 127 214534 ...... 14'69 ...... 28'4 



127 213454 ...... 14-18 ...... 35'1 



-- 127 213230 ...... 14-94 ...... 30'2 



- 127 214850 ...... 14-77 ...... 33'6 



* There can be little doubt that there is an error of 10" in these from error in the 

 number of micrometer revolutions. 



