May, 1890. the motion of the cilia of animalcula. 115 
the earliest observers, and, as they pointed out, what we see 
is not the cilia themselves, but a sort of shadow or spectre 
caused by their passing in front of each other in succession. 
A similar phenomenon is frequently seen in railway travelling 
when passing by a double row of iron railings. The pales in 
the further row, being at a greater distance, do not appear to 
coincide with those in the front row, except at intervals, and 
the result is that as the observer is carried rapidly past them 
he sees, as it were, the ghost of a set of railings much 
broader and farther apart than the real ones. 
So it is with the animalcules; probably a good many people 
here have never seen the real cilia, and have no idea how 
close they are together. 
The problem, then, is : How can we study this movement 
so as to see the action of the cilia themselves ? 
We may do it by leaving the creatures for a long time upon 
the slide without renewing the water. In plain English, by 
partly suffocating them. Or we may administer a homoeo¬ 
pathic dose of alcohol or chloroform, which will in many cases 
so reduce the rate of movement that it can be followed with 
the eye. But, in so doing, we have not completely solved the 
problem, for we are not sure that their behaviour, like that 
of more highly organised beings, may not be somewhat 
different when in liquor, and we have left unanswered the 
interesting question as to how fast they are in the habit of 
moving when sober. 
I come now to the apparatus. Most people are familiar 
with the zoetrope. It is a toy in which a number of pictures 
(as, for instance, of a tumbler in the successive positions he 
assumes in the act of turning a somersault) are presented one 
after another to the eye. The pictures are placed inside a 
revolving drum, pierced with slits, through which the specta¬ 
tors see them in succession. Now, it is well known that the 
human eye retains for a short time the impression of any 
image formed upon the retina, so that if these images follow 
one another, as in the zoetrope, in sufficiently rapid succession, 
they are combined in our perception of them into one image, 
which, in this case, appears to move. What we require now, 
is to solve the converse problem and to obtain successive 
glimpses of a moving body, so that it may seem to stand still 
or nearly so. Let us suppose we have a clock with only a 
minute hand, and that once every hour we glance at it; the 
hand will be always in the same place and will appear to have 
stood still. But suppose that the clock, instead of keeping time, 
gains five minutes in the hour. After one hour, it will seem 
to have moved five minutes; and in two hours, ten ; and 
