72 



aperture, he will see a similar strip at the bottom of the picture ; 

 consequently, in the intermediate positions of the aperture, the other 

 parts of the picture will have been projected on the retinse. The 

 width of these strips is determined by that of the small ends of the 

 apertures, which measure '125 inch ; and the diameter of the large 

 ends is 1 '5 inch, the lenses being distant 9 inches from the pictures. 

 The picture-drum being caused to revolve with the requisite rapidity, 

 the observer will see the steam-engine constantly before him, its 

 position remaining unchanged in respect of space, but its parts will 

 appear to be in motion, and in solid relief, as in the veritable object. 

 The stationary appearance of the pictures, notwithstanding the fact 

 of their being in rapid motion, is brought about by causing their 

 corresponding parts to be seen, respectively, only in the same part 

 of space, and that for so short a time that while in view they make 

 no sensible progression. As, however, there is an actual progression 

 during the instant of vision, it is needful to take that fact into 

 account in order that it may be reduced as far as practicable in 

 regulating the diameter of the eye-cylinder, and of the apertures at 

 their small ends ; and the following are the numerical data involved 

 in the construction of an instrument with the relative proportions 

 given above : 



The circumference of picture-drum=22'5 inches (A). 



The circumference of eye-cylinder=12 inches x 4 revolutions=48 

 inches (B). 



The diameter of apertures at large ends= 1*5 inch (C). 



The diameter of apertures at small ends='125 inch (D). 



While the large end is passing the eye, the picture under view 



progresses *-| ^ of 22'5 (A), or -/03 inch. 



This amount of progression (703 in.), if perceived at one and the 

 same instant, would be utterly destructive of all distinctness of defi- 

 nition ; but it is evident that the total movement brought under 



visual observation at any one moment is - Q) of -703 inch, or 



058 inch. This movement must necessarily occasion a corresponding 

 slurring, so to speak, of the images on the retina ; and the fact of 

 such slurring not affecting, to an appreciable extent, the distinctness 

 of definition, seems to be referable to a faculty which the mind has 



