32 NOTES ON 
of the eye will form an inverted image of the 
object employed. By separating one of these 
lenses, and forming an inverted telescope with 
it, using a magnifier of low power as an eye 
glass, and the eye of the insect as the object- 
glass, and adjusting their distance, a distinct 
view of objects at a moderate distance may be 
readily obtained. In this way, the focus of the 
eye may be found, as in the case of common 
lenses, if we know the exact power of the eye- 
glass:—for example, if this magnifier is the 
one-twentieth of an inch, and on looking through 
this inverted telescope at the window bars, you 
find (keeping both eyes open) that three of the 
squares of glass are exactly equal in length and 
breadth to one seen by the other eye at the 
same time without the telescope, the two images 
being brought apparently to overlap each other, 
the focal length of the eye under examination 
will be one-third of the eye-glass, or one-sixtieth 
of aninch. I regret that I have not measured 
the focal length of the eye we have been de- 
scribing, but in the common house fly (Musca 
domestica) the lenses are each about the one- 
hundredth of an inch focus. In preparing the 
compound eyes of insects, it is requisite to soak 
them for some days in water, to render them 
