322 THE PRINCIPLES OF SCIENCE. 



or moon, according as it is high in the heavens or near 

 the horizon, should be sufficient to make us cautious in 

 accepting the plainest indications of our senses, unassisted 

 by instrumental measurement. As to statements concern- 

 ing the height of the aurora and the distance of meteors, 

 they are to be utterly distrusted. When Captain Parry 

 says that a ray of the aurora shot suddenly downwards 

 between him and the land which was only 3000 yards dis- 

 tant, we must consider him subject to an error of sense 1 . 



It is true that errors of observation are more usually 

 errors of judgment than of sense. That which is actually 

 seen must be truly seen so far ; and if we correctly 

 interpret the meaning of the phenomenon, there would 

 be no error at all. But the weakness of the bare senses 

 as measuring instruments, arises from the fact that they 

 import varying conditions of unknown amount, and we 

 cannot make the requisite corrections and allowances as in 

 the case of a solid and invariable instrument. 



Bacon has excellently stated the insufficiency of the 

 senses for estimating the magnitudes of objects, or de- 

 tecting the degrees in which phenomena present them- 

 selves. ' Things escape the senses,' he says m , 'because the 

 object is not sufficient in quantity to strike the sense : as 

 all minute bodies ; because the percussion of the object is 

 too great to be endured by the senses : as the form of the 

 sun when looking directly at it in mid- day ; because the 

 time is not proportionate to actuate the sense : as the 

 motion of a bullet in the air, or the quick circular motion 

 of a firebrand, which are too fast, or the hour-hand of 

 a common clock, which is too slow ; from the distance 

 of the object as to place : as the size of the celestial 

 bodies, and the size and nature of all distant bodies ; 



1 Loomis, ' On the Aurora Borealis.' Smithsonian Transactions, quot- 

 ing Parry's Third Voyage, p. 61. 

 111 ' Novuni Oriraimin.' 



