134 Dr. Roget's explanation of 
former case. When the inclination of the bars is consider- 
able, however, the images become more crowded, and the 
distinctness of the appearance is thereby diminished. The 
deception totally ceases when the wheel is viewed through 
bars that are parallel to the line of its motion. 
6. It is essential to the production of this effect, that a com- 
bination should take place of a progressive with a rotatory 
motion. Thus, it will not take place if, when the bars are 
stationary, the wheel simply revolves on its axis, without at 
the same time advancing : nor when it simply moves hori- 
zontally, without revolving. On the other hand, if a pro- 
gressive motion be given to the bars, while the wheel 
revolves round a fixed axis, the spokes immediately assume 
a curved appearance. The same effect will also result if 
the revolving wheel be viewed through fixed bars by a spec- 
tator, who is himself moving either to the right or left ; 
because such a movement on the part of the spectator 
produces in his field of vision an alteration in the relative 
situation of the bars and wheel. 
It is evident from the facts above stated, that the deception 
in the appearance of the spokes must arise from the circum- 
stance of separate parts only of each spoke being seen at the 
same moment ; the remaining parts being concealed from 
view by the bars. Yet since several parts of the same spoke 
are actually seen in a straight line through the successive 
apertures, it is not so easy to understand why they do not 
connect themselves in the imagination, as in other cases of 
broken lines, so as to convey the impression of a straight 
spoke. The idea at first suggests itself that the portions of 
