LECT. XX. DURATION OF IMPRESSIONS. 



Fig. 36. 





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a a'. Outline (isfiires placed in the Stereoscope: a being seen by the left eye, and a' 



by the right eye. 



b. Outline of the fiirure seen in the Stereoscope by the simultaneous perception 

 of the figures a a'. 



It consists of two inclined mirrors, upon which are 

 formed by reflection the images of the two paintings repre- 

 senting the projections of a solid body in both eyes. The 

 two images are observed by applying the eyes to two 

 openings which allow the two images formed on the mirrors 

 to be seen. 



Duration of Impressions. Among the most curious phe- 

 nomena of vision, is that of the continuance of impressions 

 received on the retina. Observe a burning coal when being 

 whirled round : if the rotation be sufficiently rapid, you 

 fancy you see a circle of fire. It is evident that this illu- 

 sion can only be explained by assuming that the sensation 

 produced by the luminous body remains for a certain time, 

 which can be estimated by the interval necessary for the 

 coal, in revolving, to return again to the same point; so 

 that we see it simultaneously at all the points which it suc- 

 cessively traverses. The apparent augmentation of volume 

 which a cord undergoes during vibration, the disappear- 

 ance of the spokes of a wheel which revolves with great 

 rapidity, the luminous train of shooting stars, and the 

 white appearance of a revolving disc, on which the seven 

 colours of the spectrum are painted, are phenomena depend- 

 ing on the same cause; that is, the duration of impressions 

 on the retina. If light were instantaneous all these phe- 

 nomena would cease. Look steadfastly for some seconds 



