xin.] MEASUREMENT OF PHENOxMENA. 277 



fatigue of his eye. The same is true of lunar oliser- 

 vations; and even the use of the best telescope fails 

 to remove this dilHculty. In judging of colours, again, 

 we must remember that light of any given colour tends 

 to dull the sensibility of the eye for light of the same 

 colour. 



Nor is the eye when unassisted by instruments a much 

 better judge of magnitude. Our estimates of the size of 

 minute bright points, such as tlie fixed stars, are com- 

 pletely falsified by the effects of irradiation. Tycho 

 calculated from the apparent size of the star-discs, that 

 no one of the principal lixed stars could be contained 

 within the area of the earth's orbit. Apart, however, from 

 irradiation or other distinct causes of error our visual 

 estimates of sizes and shapes are often astonishingly 

 incorrect. Artists almost invariably draw distant moun- 

 tains in ludicrous disproportion to nearer objects, as a 

 comparison ot a sketch with a photograph at once shows. 

 The extraordinary apparent difference of size of the sun 

 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 Pariy 

 says that a ray of the aurora shot suddenly downwards 

 between him and the land which was only 3,000 yards 

 distant, we must consider him subject to an illusion of 

 «eiise.^ 



It is true that errors of observation are more often 

 errors of judgment than of sense. That which is actually 

 .seen must be so far truly seen ; 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 w'e cannot make the 

 requisite corrections and allowances as in the case of a 

 solid and invariable instrument. 



liacon has excellently stated the insufficienc}' of the 



' Looiiiis, On ihe Aurora HoreMis. Smitlisoiiian Transact iuiis, 

 quti'iii^ I'any'H TJjini Voyage, p. 61, 



