ipii.J XIPHER— AN OPTICAL PHENOMENON. 317 



Many observers have doubtless had experiences hke those which 

 the writer had years ago while doing surve}- work. Two transit 

 instruments were available, one of which showed the object viewed 

 in erect, and the other in inverted position. A few days of use of 

 either instrument enables the observer to give proper signals to the 

 rod-man in a perfectly automatic way. After having thus become 

 alternately educated, an attempt to use these instruments at random 

 for brief intervals, relying wholly on what he sees through the instru- 

 ment for the information which is to guide him in making his sig- 

 nals, leads to the most helpless confusion. The observer even seeks 

 to find his way out of his difficulties by comparing what he sees 

 through the instrument with the impression received by a direct view. 



Such experience as this appears to justify the conclusion that we 

 see external objects as we have learned how to see them, by help of 

 our other senses. Even then it is a matter of never-ending wonder 

 that we have in our possession certain nerve-fibers that can be trained 

 to see. 



There are many interesting features of the phenomenon which 

 LeConte discussed which appear to have escaped his attention. His 

 claim that the sharp outline of the pin head seen in the hole could 

 not be an optical image, since such an image would be so much out 

 of focus as to be invisible, is justified to this extent. The object is 

 in fact also visible in its real position in shadowy outline. It appears 

 transparent, and the inverted shadow of the pin head is mentally 

 projected outward and appears to be visible through the object itself. 

 Every detail of the letters on a printed page is visible through this 

 enlarged and transparent appearance which the object itself presents, 

 due to an out-of-focus image on the retina. 



The sharpness of outline of the shadow decreases as the hole is 

 made greater in area. This is due to penumbral effects. A black 

 card gives more sharply defined results than a white one. A tube 

 having the pierced card at one end and the pin head at the other may 

 be applied to the eye, in such a way as to cut ofl:' all side light. The 

 head may be covered with a black cloth, which is also wrapped around 

 the tube. The shadows are then as sharply defined as an optical 

 image could be. If the black sateen cloth be thrown over the head, 

 and the eves be directed towards a bright skv, a multitude of cir- 



