538 Royal Society : — 



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

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

 apertures, which measure ■ 1 25 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=l'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 ! J| ^ of 22-5 (A), or 703 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 



•125 (D) 

 visual observation at any one moment is -j— >p< 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 

 of correcting or disregarding certain discrepant appearances or ir- 

 regularities in the organ of vision ; as a further illustration of which 

 I may cite the fact, mentioned by Mr. Warren De la Rue in his 

 " Report on Celestial Photography," that the retinal image of a star 

 is, at least under some atmospheric conditions, made up of " a great 

 number of undulating points," which, however, the mind rightly 

 interprets as the effect of the presence before the eye of a single 

 minute object. That this corrective power is, as might be supposed, 

 very limited, may be proved experimentally by this instrument ; for 

 if the small ends be enlarged in only a slight degree, so as to increase 

 this slurring on the retinae, a very marked diminution in clearness of 

 definition is the immediate result. 



That form of the stereotrope, in which Professor Wheatstone's re- 

 flecting stereoscope is made use of, and w r hich is better adapted for 



