Disks to the Optical Analysis of Vibrating Bodies. 21 



suited ; for with feebly lighted objects the pupil is enlarged to 

 something beyond the normal measure. 



It follows from this that holes of smaller diameter than that 

 of the pupil are of no use to the distinctness of the picture, 

 since they, staying with their full aperture in front of the 

 pupil, leave a constant quantity of light in the eye, which in this 

 case does not attain a momentary maximum. But if, with very 

 high tones, it be desired to use smaller holes in the disk B, 

 there must be in front of the eye just so small a stationary 

 aperture as to obtain a momentary maximum of intensity. 

 Hence close before the rotating disk B (fig. 1) is a smaller 

 disk G, which is provided with apertures of very different sizes, 

 corresponding to the different rotating series of holes. This 

 disk G, by means of a stand E t moveable about t } may be 

 fastened in front of each series of holes, so that the corre- 

 sponding apertures of G and B in a certain position cover one 

 another. The observing eye is placed close in front of the 

 corresponding stationary aperture of G. 



What has been said above respecting circular apertures ap- 

 plies, of course, to radial slits in the disk B. It is useless also 

 to make them narrower than the diameter of the pupil (if they 

 are observed without the disk G); for by this means the bright- 

 ness and distinctness are limited in the same direction. But if 

 in the stationary disk G an equally large slit is made which 

 may be accurately placed in front of the rotating series of slits 

 in B, it presents many advantages. For, in the first plaee, the 

 conditions for the occurrence of* a momentary maximum bright- 

 ness are more favourable in the case of two parallel slits slid over 

 one another than in that of circular apertures, as is shown by a 

 simple calculation. On the other hand, a larger portion of the 

 active aperture of the pupil can be used than with small circular 

 holes. Hence for my experiments I have used paper disks, B, 

 with the following numbers of holes, of which each three are in 

 one disk :— 2, 3, 4; 6, 9, 12 ; 18, 24, 36. The last three series 

 of holes are slit- shaped, the width of the slit being 2 millims. 

 and 1^ millim. ; the others are circular holes of 3 to 4 millims. 

 diameter. Smaller slits than those give distorted images, owing 

 to phenomena of diffraction. The same series are, of course, all 

 represented on the observing-disk G. 



As a matter of course the total brightness of the moving 

 images is far smaller than the brightness of ordinary vision ; and 

 it can be readily seen that with equal magnitude of the holes it 

 increases with increasing number of holes. For, supposing the 

 series of holes formed an open circular band of the breadth of 

 the diameter of the pupil, the brightness of the image would 

 clearly be equal to 1. But if the circular band is interrupted, 



