Dr. Pearson's Introduction to Practical Astro7wmy. 373 



Moreover, the precision in the construction of astronomical instru- 

 ments is now brought to so high a degree of perfection, that it 

 requires most cautious and delicate treatment in the use of them 

 to do them justice. In the construction of the old instruments, in 

 which the imperfections resulting from inaccurate divisions, or bad 

 workmanship in general, were much greater than those resulting 

 from the want of proper precautions on the part of the observer, 

 these last were not likely to be detected : indeed it would have been 

 useless to attempt to detect them j for so long as they were less 

 than the instrumental errors, their application, as equations, must 

 have been in all cases vague and uncertain. The main object was 

 then, as it must be 7101x1, to use such precautions as will keep the 

 errors of observation below the errors of construction; and this, 

 perhaps, in the old instruments, such as the great mural quadrants, 

 was not difficult to effect. But the case is far different with instru- 

 ments of modern date and construction; for should an observer be 

 furnished with instruments of the most perfect workmanship, and 

 should he, from not taking the proper precautions in making his 

 observations, introduce errors which are of greater magnitude than 

 the quantities which are the objects of his search, all his labours 

 must prove worse than worthless. The perfection of modern in- 

 struments is such as to require the greatest care and skill on the 

 part of the observer, to do full justice to the powers of his instru- 

 ment. As an illustration of this statement we may refer to the con- 

 troversy on the subject of annual parallax in certain fixed stars, 

 which was for some time warmly kept up between the present As- 

 tronomer Royal, and the late Professor of Astronomy in Dublin : 

 nor is the question yet settled; nor need this be a matter of sur- 

 prise, when we reflect that the absolute quantity in dispute lies 

 within a single second of space* ! 



In order that any instrument may have its full powers brought 

 into action in the best manner, it is necessary that extreme caution 

 should be used by the observer : for, independently of errors arising 

 from the want of due adjustment of the instrument, there are errors 

 originating in the observer himself, which a person accustomed to 

 make observations will readily understand. Another source of error 

 arises from imperfect division of the limb ; but in modern instru- 

 ments this is almost annihilated : yet, minute as it remains, it is neces- 

 sary that the observer should keep the error resulting from the first 

 sources less than this last-mentioned error. To do this requires all 

 his powers and attention : he must direct every effort towards pre- 

 cision in completing the requisite adjustments, and also in making 

 his observations ; so that the errors originating in himself may ge- 

 nerally, or always if possible, be less than the errors of his instru- 

 ment. Unless he does this, however perfect the instrument may 

 be, he can never do full justice to its powers; a less perfect instru- 



• Mr. Pond's paper on tlie parallax of a. Lt/ra:, in which part of this 

 discussion is contained, will be fotnul in Piiil. Ma^. vol. Ixii. p. 2i)l^; and 

 further information on the subject at p. 452 and 466 of the same volume. 



