Mr. Milward on the Action of Light on Revolving Discs. 451 



that if the last-mentioned time be equal to the period in which 

 the wheel revolves, all the positions of the wheel (m) will be 

 impressed upon the retina at once, and the confusion will be 

 very great. Thus when the motion is slow, the action on the 

 eye may be considered as caused by groups ofmth. positions* 

 whose duration is n", and each of which gives an indistinct 

 and complicated image. The length of these groups is evi- 

 dently the space passed through during the time of the dura- 

 tion of the first image on the retina. As the motion therefore 

 becomes slower a less number of mth positions is contained 

 in each group, the length of the group (or the range of con- 

 fusion) becomes less, and the confusion decreases. We may 

 here perhaps mention the operation of another cause which 

 interferes with the principle we have been considering f. 



We have in the former part of the subject supposed the 

 axis of the eye to be stationary ; but we find by experience 

 that this is not the case. When a wheel moves very slowly 

 the eye follows some particular part which attracts attention, 

 so that by the spontaneous action of the muscles of the eye the 

 axis of the optic lens is always directed towards. that particu- 

 lar part, and the images may be reflected to it nearly in the 

 same manner from each portion (according however to the 

 method in which the experiment is conducted). Both the eye 

 and the wheel being thus in similar motion, the effect is in 

 some degree the same as if the wheel were at rest. This dis- 

 turbing influence appears to exist in all experiments in which 

 motion is admitted, but must from necessity decrease very 

 rapidly as the velocity of the wheel becomes greater. 



We will now consider the effect of light upon revolving 

 wheels, when the duration of the former is very short. In 

 this case we perceive clearly that if the light consist only of 

 one system (such as we have supposed), or in other words, do 

 not exceed the time of the mth part of a revolution, one image 

 only will be reflected to the eye, which, according to our pre- 

 vious hypothesis, will be accompanied with only a slight de- 

 gi'ee of confusion ; and thus we have the general principle, 

 that a light of sufficiently short duration produces a clear 

 image of the entire revolving wheel, and at the same time that 

 as the duration of light increases, the number of images of 



* By the term mth position is meant the group of positions contained in 

 the with portion of a revolution, and reflecting nearly the same image to 

 the eye. 



t An interesting illustration of the confusion ahovc mentioned, is af- 

 forded in a railway train moving along a raised embankment when the cir- 

 cumferences only of the wheels are distinguishable, and the wheels tiuis 

 partially teen appear at rest, and to be gliding along tiie line. 

 2 H 2 



