THE YOUNG SCIENTIST. 
65 
How a Fly Sees. 
AMONGST the objects which are to be 
found in every little collection is the 
fly's eye, so called. On placing it under 
the microscope, and bringing it into focus, 
so as to get a clear view, we see an object 
like a piece of netting, with six-sided 
meshes. And to most persons this is the 
end of the fly's eye. They are told that 
each mesh is a veritable eye, and they 
believe it, because they have faith in what 
scientiflc men tell them. But they have 
BO clear ideas on the subject. If we look 
at the head of a fly, and particulary that 
EYE OF FLY. 
of a large blue-bottle or a dragon fly, w^e 
shall see that it consists chiefly of two 
large round lumps, which, under a good 
magnifier, are seen to be covered with 
facets, each one of which is an eye, and it 
is these eyes that under the compound 
microscope are seen as the meshes of the 
apparent net. Each one of these eyes 
transmits a picture or image of all sur- 
rounding objects within their scope, but it 
does not thence follow that the fly sees a 
thousand faces any more than that we see 
all objects doubled when we look at them 
w^ith our two eyes. 
To prove that each of these facets is a 
•distinct eye is not a very difficult matter. 
Those who have good microscopes and a 
little experience, find no difficulty in 
showing an image of any object in each 
facet, so that the whole field of view is 
filled with little pictures of whatever ob- 
ject may be selected. The most striking 
object is perhaps a watch face with the 
second hand in motion. We then seem 
to see a thousand watch faces, in each of 
which the seconds are ticked off with the 
most perfect harmony. To show the 
watch face clearly requires strong illu- 
mination and good definition. In any 
case, however, the proper illumination of 
the object is the chief point necessary to 
secure success. Therefore, it will be found 
that the easiest object on which a beginner 
can try his hand is the flame of a lamp or 
candle. Arrange the microscoi)e in an in- 
clined position, with a 2-3 or 1-2 inch ob- 
jective, and place a candle or taper below 
the stage, and about two inches from it. 
If the microscope be sufficiently inclined 
the heat and flame will cause no annoy- 
ance. The lamp must be in the direct line 
of the optic axis of the microscope ; that 
is to say, a line passing through the 
centre of the body of the microscope 
ought to pass through the flame of the 
candle. The fly's eye, if now placed on 
the stage, w^ill be illuminated and may be 
focussed so as to show the meshes clearly 
and sharply. If we now move the objec- 
tive away from the object we will find, 
after a little while, that we have a distinct 
view of the flame of the candle in every 
mesh or facet, and if a breath of air should 
stir the flame, the hundreds of little 
flames seen in the field of view will move 
in precisely the same direction and at the 
same time. 
This is one of the most interesting and 
instructive microscopical experiments 
that we know of, and if the reader will 
study the conditions of success by a little 
experimenting, he will soon be able to 
give good exhibitions of the most difficult 
subjects. 
Scientific Amusement. 
UNDEK this head we propose to give 
a series of articles containing direc- 
tions for performing simple but striking 
experiments in various branches of 
science. In selecting experiments for 
this department great care will be taken 
to choose those only which are entirely 
free from danger, either to the person. 
