320 Transactions of the Boyal Microscopical Society. 
sunlight and gave the appearance of a broad golden band suspended 
in the air above the tower. The vertical diameter of this band 
far exceeded the optical angle nnder which the vanes would have 
appeared if it had been possible to see them correctly with the 
naked eye ; hut there was no error of position, and the grouping of 
the vanes in one unbroken line simply arose from the distance 
being too great for the interspaces between them to be noticeable. 
In such a case what is seen ? Certainly not the vanes, hut a well- 
defined optical image differing considerably from them. The non- 
correspondence of this image with fact was inferred partly from 
the o&ervers having seen the vanes at a short distance, and partly 
from the improbability that anyone had suspended a broad gilt 
band in such a situation. Had there been no knowledge of the 
real structure and no improbability in there being a broad gilt 
band over the tower, the optical appearance might have led to a 
belief in its existence. May we not suppose that microscopists are 
sometimes in danger of being misled by appearances they have no 
means of bringing to any decisive test ? 
