142 Mr. S. Bidwell. On the Recurrent [June 7, 



able to detect the existence of a second, and even of a third, dark 

 band of greatly diminished intensity, though he adds that the obser- 

 vation is a very difficult one.* What is probably the same effect in a- 

 different form can, however, be shown quite easily in the following 

 manner. 



Experiment 7. 



In a blackened zinc disk 15 cm. in diameter, there were cut two 

 opposite radial slits, about 0'5 mm. in width. The disk was rotated 

 at the rate of one turn per second in front of a sheet of ground glass, 

 behind which was an incandescent lamp. The glass was covered 

 with opaque paper, in which a circular opening was made of slightly 

 less diameter than the disk. The disk was placed opposite this 

 opening, and no light reached the eye except such as passed through 

 the two slits. When the disk was observed from a distance of about 

 1^ metres, the eye being fixed upon its centre, each slit 'appeared to 

 give four (or possibly five) luminous images, arranged like the ribs 

 of a partly opened fan. The images were distinctly separated by 

 dark intervals near the circumference, but overlapped one another 

 towards the centre. The leading image was naturally the brightest, 

 each consecutive image being considerably weaker than its precursor. 

 All had the same tone of colour, namely that of the yellowish-light 

 given by the electric lamp. The usual blue recurrent image could 

 also be seen following the images of the radial slits, at an angle of 

 about 80. 



It appears, then, that when the retina is exposed, to the action of 

 light for a limited time, the complete order of visual phenomena is 

 as follows : 



(1) Immediately upon the impact of the light there is experienced 

 a sensation of luminosity, the intensity of which increases for 

 about one-sixtieth of a second : more rapidly towards the end 

 of that period than at first. 



(2) Then ensues a sudden reaction, lasting also for about one- sixtieth 

 of a second, in virtue of which the retina becomes partially 

 insensible to renewed or continued luminous impressions. 

 These two effects may be repeated in a diminished degree, as 

 often as three or four times. 



(3) The stage of fluctuation is succeeded by a sensation of steady 

 luminosity, the intensity of which is, however, considerably 

 below the mean of that experienced during the first one-sixtieth 

 of a second. 



* I Lave noticed that the intensity of the dark band always appears to fluctuate 

 very rapidly, perhaps twenty or thirty times in a second. The rate of fluctuation 

 is quite regular, and independent of the rate of rotation. 



