Rambaut — On the Position of Enckes Comet. 571 



which measures are made, a', S', those of another star, then the axis of 

 y being the projection of the meridian passing through the centre, 

 and X, y the rectangular co-ordinates of the image of the second 

 star, then it is shown, on p. 236, that 



y cos S sin 8' - sin 8 cos 8' cos (a' - a). 

 X cos 8' sin (a' - a) 



This is the tangent of the angle (^) which the line joining the 

 stars makes with the axis of x. To orient the plate in the microscope, 

 therefore, we have only to place the plate so that a movement of one 

 of the two screws will carry the cross-wires in the eyepiece from one 

 star-image to the other. The line joining the star-images is then 

 parallel to this screw. IN^ext, by means of the position circle, the 

 plate is turned through the angle <^. It is then set correctly for 

 measiu'ement from the centre. 



In the case before us the positions of the stars being given by 

 Argelander for 1855, the H-screw was set in the parallel passing 

 through +7°, 4887 for that year, so that the necessity for correct- 

 ing the co-ordinates of the stars for precession, to bring them up 

 to 1894, was avoided. In this way the co-ordinates of the comet 

 were referred to the equinox and equator of 1855, and then the 

 precession corrections applied finally to bring up the place of the 

 comparison star to the date of observation. 



The appearance of the comet on the plate was not sharply defined 

 in outline, nor was there any distinctly marked nucleus from which 

 measures of a very high degree of precision could be made. It 

 showed, however, a gradual condensation towards the centre, and the 

 densest part of the image was accordingly taken as the point for 

 measurement. 



Under the circumstances it is of course unnecessary to take in the 

 smaller terms in the formulae of reduction, and we have simply 



Aa cos 8 = RhS (1 + i?^r sin 1" tan 8), 



and A8 = i2j,F- i R-jjE- sin I" tan 8, 



in which R^ denotes the value of a revolution of the ^-screw, 



Ry ,, „ ,, F-screw, 



Aa ,, the difference in E.A. of the two stars, 



A8 ., ,, in declination of the two stars, 



and J?", V, the corresponding measures expressed in revolutions of the 

 screws. 



