Brunnow — On the Parallax of the Plane^iri/ Nebula. 125 



XX. — Discussion of Observation's por determining the Parallax 

 OF the Planetary Nebula, 37, H. IV. Made with the South 

 Equatorial at Dunsink. By Francis Brunnow, Ph. D., F. R. A. S. 



[Read, November 12, 1877.] 



At the Meeting of the British Association for the Advancement of 

 Science, held at Edinburgh in 1871, Mr. Gill read a Paper on the 

 "Parallax of the Planetary jN^ebula H. IV., 37," for which he had found 

 a value of about two seconds. However, the number of his observa- 

 tions was so small, that it seemed to me advisable to make a longer 

 series of observations of this interesting object, in order to examine 

 whether such a large parallax really existed. The observations were 

 commenced immediately after my return home on August 13, 1871, 

 and were continued to August 6, 1872, with some interruptions 

 owing to my absence from the Observatory during the months of 

 January and February, 1872, and again during part of April and May. 

 They are, therefore, not as numerous as I could have wished, but still 

 are sufficient to show that the nebula has no large parallax. 



The nebula appears as a somewhat elliptical disk whose major 

 axis is about half a minute, and has in its centre a well-defined point 

 resembling a star of the eleventh magnitude. I compared this centre 

 in declination with a star of the tenth magnitude which precedes the 

 nebula by 25 seconds, using exactly the same method of observing 

 as that adopted in my former series of observations on the parallax of 

 stars. I also used a faintly illuminated field, as I could make the 

 bisections of these faint objects more accurately with dark wires than 

 in a dark field with bright wires. Of course, I observed only when 

 the atmosphere was sufficiently good to show the central point dis- 

 tinctly. 



The observations I have obtained are as follows : — 



Date. 



a5 expressed 



in rev. of the 



screw. 



Therm. 



A5 in 



seconds. 



Weight. 



1871. 











August 13, 



6-95815 



55°-0 



62"-561 





15, 



6-95670 



54 -0 



62 -551 





25, 



7-02535 



50 -5 



63 -176 





27, 



6-98780 



52 -0 



62 -834 





September 11, 



6-99955 



53 -0 



62 -939 





12, 



6-98115 



54 -0 



62 -773 





13, 



6-98390 



52 -0 



62 -799 





21, 



6-98320 



42 -0 



62 -813 





