THE EXACT MEASUREMENT OF PHENOMENA. 321 



of the light received, and thus admits more or less light 

 according to circumstances. The moon which shines with 

 almost dazzling brightness by night, is pale and nearly 

 imperceptible while the eye is yet affected by the vastly 

 more powerful light of day. Much has been recorded 

 concerning the comparative brightness of the zodiacal 

 light at different times h , but it would be difficult to prove 

 that these changes are not due to the varying darkness 

 at the time, or the different acuteness of the observer's 

 eye. For a like reason it is exceedingly difficult to esta- 

 blish the existence of any change in the form or compara- 

 tive brightness of nebulae ; the appearance of a nebula 

 greatly depends upon the keenness of sight of the ob- 

 server, or the accidental condition of freshness or fatigue 

 of his eye ; the same is true of lunar observations ' ; and 

 even the use of the best telescope fails to remedy this 

 difficulty. 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 the fixed stars, are com- 

 pletely falsified by the effects of irradiation. Tycho calcu- 

 lated from the apparent size of the star- discs, that no 

 one of the principal fixed stars could be contained within 

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

 tion or other distinct causes of error, our visual estimates 

 of sizes and shapes are often astonishingly incorrect. 

 Artists almost invariably draw distant mountains or other 

 objects in ludicrous disproportion to nearer objects, as a 

 comparison of a sketch with a photograph at once shows, 

 The extraordinary apparent difference of size of the sun 



h 'Cosmos,' Translated by Ottd, vol. i. pp. 131-134. 

 i 'Report of the British Association,' 1871, p. 84. Grant's 'History 

 of Physical Astronomy,' pp. 568-9. 



Y 



