OF ARTS AND SCIENCES. 195 



times, and many from thirty to forty times. The second series is a 

 portion of the great zone observations now going on under tiie charge 

 of the Astronomische Gesellschaft of Leipzig. The rules for tliis 

 series were formulated by Argelander. 



The sources of error (not mistakes) in astronomical observations are 

 partly psychological (deficiency of attention), partly psycho-i)liysic:iI (dul- 

 ness of the senses, time expended in communications through the nerves), 

 partly instrumental, or depending on temperature, partly optical, depend- 

 ing on the condition of the atmosphere. Tlie first rule then is that the 

 observer must keep himself in uniform condition, and therefore be tem- 

 perate and regular in his life. He must keep his senses constantly 

 under control. He must have good instruments, well and firmly 

 mounted : all the parts of the instruments must be solid. The differ- 

 ent instruments must correspond with each other in their degree of 

 perfection, and must always be in good repair. Observations must 

 not be made when the air is uncommonly disturbed, or when the 

 observer cannot keep himself warm enough to be comfortable, — of 

 course when it is practicable to make the observations at all under 

 better circumstances. 



The single observations should be uniform ; i. e., on nearly the same 

 number of wires, with nearly the same number of settings and micro- 

 scope readings. 



Too great fatigue should be avoided by timely pauses, so that, for 

 instance, the first observations of a night may not be very good, and the 

 last very bad. 



Long series of observations have been greatly damaged by the fol- 

 lowing causes, among others : — 



Large errors of division, much exceeding the errors of setting upon 

 a star. 



The wearing at the centre of a quadrant; and a gradual flexure of the 

 whole instrument. 



Placing a transit instrument in a high tower, which expanded and 

 contracted by the sun's heat. Too great trust in the fixity of an 

 instrument. Closely counterpoising a meridian circle, so that, 

 when observing zones in a hurry, the axis was lifted out of its 

 bearings. 



"Weakness of the telescope tube, and an attempt to improve it by levers 

 of flexure. 



