xlviii 



INTRODUCTION TO THK 



Applying these formulas to the preceding observations, the instrumental deviations at the 



successive dates were as follows : 



Instrumental deviations. 



These results show that the deviations of the equatorial could have no appreciable influence 

 on observations made as were the differential measures with the planets Mars and Venus. 



Value of the Micrometer Screw. 



To determine the value of the micrometer-screw, transits of ft Hydri were observed over the 

 index and micrometer wires, placed at intervals of five, ten, and twenty revolutions apart. 

 They gave the following results at different periods : 



The value adopted for the reduction of observations is 



19".49. 



The diameter of the micrometer-wires was first ascertained by turning the telescope towards 

 clear sky during day-light, and bringing the movable wire in contact with the index-wire alter- 

 nately on each of its sides. Under a magnifying power of 235, the index-wire appears some 

 what the greater of the two, but they formed part of the same fibre, and the difference between 

 them could not be detected when superposed. A mean of ten measures by day-light gave for the 

 diameter of the two 2". 504, or, on the assumption of equality, each wire had a diameter of 1".252. 

 Observations by lamp-light, however, give a very different result. Thirty measures, made at 

 different periods, show that the effect of the lamps is to increase the apparent diameters from 

 1".252 to 2". 199. In the reduction of all observations after lamp-light, the diameter adopted is 



2". 20. 



Frequent occasions occurred, during the progress of the adjustments, to test the optical 

 capacity of the instrument. The first object viewed with it was the planet Saturn ; and though 

 twilight had scarcely closed in, five of the satellites, the shadows of the rings, and dark inte- 

 rior spaces, were admirably shown under a magnifying power of 235 times. The sixth star of 



