THE EYES OF SCIENCE. 299 



contrast. Thus, the French astronomer Chacornac 

 advanced a very ingenious indeed, masterly theory 

 in explanation of the circumstance that the disc of the 

 planet Jupiter is brighter near the edge than in the 

 middle, the only objection to his theory residing in the 

 circumstance that the disc is darker, not brighter, near 

 the edge, though to the eye it appears brighter there 

 by contrast with the dark background of the sky on 

 which it is seen projected. So again there is a 

 charming theory, in vogue to this day among many 

 students of the moon, explaining why the floor of the 

 lunar crater Plato (the Greater Black Lake of the 

 earlier telescopists) grows darker as the sun pours more 

 light upon it (rising higher in the sky as supposed to 

 be viewed from Plato), the real fact being that there 

 is no such darkening, the apparent difference being 

 entirely due to effects of contrast the contrast of the 

 floor with the black shadows of the crater-ring thrown 

 upon it when the sun is low, and the contrast of the 

 floor with the brilliant white of the surrounding crater- 

 ring when the sun is high, one contrast making the 

 floor look lighter than it is, while the other makes it 

 look darker. I may cite another instance of an optical 

 illusion, caused by an effect of contrast a case not 

 requiring telescopic observation for its recognition. If 

 on a moonlit night one looks beyond a water horizon 

 towards the part of the sky below the moon, that 

 region looks darker than the parts of the sky on either 

 side ; yet, in reality, it is no darker if anything 

 slightly lighter. What causes it to look darker is the 



