128 SPECIAL SENSES. 



brief duration of an electric spark, it appears absolutely sta- 

 tionary, as the period of time necessary for perfectly distinct 

 vision and the duration of the illumination are so short, that 

 there is no time for any appreciable movement of the object. 

 The familiar experiments made with revolving disks beauti- 

 fully illustrate these points. In a disk marked with alter- 

 nate radiating lines of black and white, the rays become en- 

 tirely indistinguishable during rapid revolution, and the disk 

 appears of a uniform color, such as would be produced by a 

 combination of the black and white. Yery beautiful effects of 

 artificial combination of colors may be produced in this way, 

 the resultant color appearing precisely as if the individual 

 colors had been ground together. It is also interesting, in this 

 connection, to note that the duration of the retinal impres- 

 sions varies considerably for the different colors. This fact 

 was observed some years ago by Plateau, 1 and was confirmed 

 by Emsmann, 9 there being some slight variations in the com- 

 parative results obtained by these two experimenters. Ac- 

 cording to Emsmann,' the duration for yellow is 0'25 of a sec- 

 ond ; for white, O25 of a second ; for red, O22 of a second ; 

 and for blue, O21 of a second. 



It is unnecessary to describe farther in detail the well- 

 known phenomena which illustrate the point under considera- 

 tion. The circle of light produced by rapidly revolving a 

 burning coal, the track of a meteor, and other illustrations 

 are sufficiently familiar, as well as many scientific toys pro- 

 ducing optical illusions of various kinds. 



Irradiation. It has been observed that luminous impres- 

 sions are not always confined to the elements of the retina 

 directly involved, but are sometimes propagated to those im- 

 mediately adjacent. This gives to objects a certain degree of 



1 PLATEAU, Ueber einige Mgenschaftcn der vom Lichte auf das Gesichtsorgan 

 hervorgebrachten Eindrucke. Annalen der PhysiJc und Chemie, Leipzig, 1830, Bd. 

 xx., S. 309. 



2 EMSMANN, Ueber die Dauer des Lichtendrucks. Annalen der Physik und 

 Chemie, Leipzig, 1854, Bd. xci. (vierte Reihe, Bd. i.), S. 616. 



