398 THE POPULAR SCIENCE MONTHLY. 



server reads the microscopes of a Transit Circle habitually too large, 

 when he is determining the zenith-distance of a star, it is likewise 

 his habit to read them too large when determining the position of the 

 zenith-point from which zenith-distances are counted ; and the re- 

 sulting quantity is likely to be free from all but accidental errors. 



Occasionally there arise cases where these differences (in the same 

 observer) are not eliminated, but multiplied. 



In the measurement of a base-line, for example, the various rods 

 are brought into contact under a microscope : if an observer judges 

 these rods to be in contact when they are not, it is evideut that his 

 error, originally small, will augment with the number of contacts, and 

 it may become serious. 



In the comparison of the national standards of length, undertaken 

 by the English Ordnance Survey, an annoying case of personal differ- 

 ence was found. 



These comparisons were made by bringing a movable cross of spi- 

 der-lines to bisect one of the lines engraved, on the various bars, and 

 it was found that Captain Clarke, R. E., and Quartermaster Steel, 

 R. E., who made the greater number of comparisons, differed in their 

 estimation of a bisection by a constant amount which was annoyingly 

 large : so that " the probable error of the final results is nearly double 

 what might be expected from errors of observations only." This error 

 cannot be eliminated, and it still remains in the published results. 



We must constantly bear in mind that the quantities of which we 

 have all along been speaking are extremely small, and that in fact 

 they are masked by accidental errors for inexperienced observers in 

 most cases. Still they exist, and they are among the most curious of 

 phenomena : their careful study would well repay physiologists. 



We can never be sure we have eliminated them so long as the hu- 

 man mind or body is a part of the machine by means of which we 

 are comparing or registering events ; and, just so long as mind or body 

 is employed, we can be sure that personal differences will not only 

 exist, but that they will vary from day to day. We must use for 

 eliminating personality those values which are the best attainable, 

 and assume these values to be constant over extended periods of time 

 weeks or months. In astronomy of precision, however, we have 

 other errors to fear much more variable than personal equation, and 

 it is to the elimination of these that attention should be directed. In 

 other branches of research less exact in method, personality becomes 

 of more importance, and an attentive consideration of its effects may 

 be well worth while undertaking. 1 



1 The writer has recently had occasion to examine drawings of the same nebula by 

 different observers, with telescopes which are quite similar, and the enormous differences 

 which exist in the representations show personal differences of the most marked kind, 

 for nothing is more certain than that all the changes shown by the drawings have not 

 taken place. 



