10 



Proceedings oftJie Royal Ii'ish Academij. 



no system of formula) seemed to me so convenient as tliat suggested by 

 Dr. Eambaut, which I have accordingly employed.^ I have hitherto 

 only measured one plate of the nebula !M. 33, taken at Dr. lloberts' 

 observatory, Crowborough, Sussex, on the 27th December, 1899, the 

 exposure being made from 2^ 33"' to 4'' 3"", Sid. Time. 



According to the method of reduction employed, we take a pair of 

 known stars, or, better still, two pairs of stars, the right ascensions 

 and declinations of which are known from meridian observations, and 

 compute their standard coordinates ^ and r/, taking refraction and 

 aberration into account. The corrections for refraction are of the form 



= - slj}^ + qn], dv = - ^[q^ + 



where the logarithms of the constants have the following values for 

 the plate in question : — 



logp = 9-9271 log r = 9-9618 



log q = 9-0572 log s = 6'5782 



The corrections for aberration are 



di = [5-925]^, drj = [5-925] 77. 



The following table gives the means of finding the corrections 

 without computation : — 







Refraction. 





Aberr. 



1 01-77 









d^^ ox 

 dr) 





s q ^ or 

 sq n 



S 1-7] 



100" 



0"-032 



0"-004 



0"-035 



0"^008 



200 



•064 



-009 



•069 



•017 



300 



•096 



•013 



•104 



•025 



400 



•128 



•017 



•139 



•034 



500 



•160 



•022 



•173 



•042 



GOO 



•192 



•026 



•208 



•050 



TOO 



•224 



•030 



•243 



•059 



800 



•256 



•034 



•278 



•067 



900 



•288 



•039 



•312 



•076 



1000 



•320 



•043 



•347 



•084 



1100 



•352 



•048 



•381 



•092 



1200 



•384 



•052 



•416 



•101 



1300 



•416 



•056 



•451 



•109 



1400 



•448 



•061 



•486 



•118 



1500 



•480 



•065 



•521 



•126 



1600 



•512 



•069 



•556 



•134 



1700 



•544 



•073 



•590 



•143 



1800 



•576 



•078 



•624 





1 Monthly Notices, E.A.S., vol. Ivii., p. 591. 



