294 THE NORTHERN MICROSCOPIST. 
and even then, our judgment being formed by comparison, and 
also by experimental contact of substances with our senses—so to 
speak—opinions which are formed must, to a certain extent, be 
modified with the amount of true experience to which our nerve 
centres have been previously subjected. 
Take two experts ; give to each one a sphere composed of lead 
and tin. Upon asking them what substance they were handling they 
might probably guess, perhaps not; they would poise it in their 
hands, look at it, smell it, try to cut it, perhaps, examine its metallic 
lustre, and it would be very odd indeed if they could agree as to 
the composition of the alloy, unless settled by an assay upon the 
balance. 
Has it ever struck any one here that such processes as these go 
on in Microscopy, and that it is necessary to carefully study the organ 
of vision in order to gain a true insight into the object presented 
to us? 
I will now throw upon 
the screen a diagram of 
the eye (fig. 22), after 
Helmholtz. It is, as you 
will see, a nearly spheri- 
cal ball, capable of many 
movements in its sockét. 
It possesses an outer 
translucent covering 
called the sclerotic coat, 
or simply sclerotica, which 
: may be seen at H. It 
Fig. 22. is thick, horny, and 
opaque, except in its anterior portion. 
This sclerotic coat envelopes about § of the eyeball, and in 
common parlance is called the white of the eye. 
The anterior transparent portion is called the cornea, and has 
the shape of a very convex watch glass. It is through this mem- 
brane that the light passes to the interior of the eye. The cornea 
and the anterior portion of the sclerotica are covered with a 
mucous membrane. 
_ Behind the cornea is a diaphragm of annular form called the 
iris; it is coloured and opaque, the circular aperture in its centre, 
C, being called the pupil. 
_ The tris, D, serves the purpose of regulating the admission of 
light; it varies in colour in different individuals, and is the part 
referred to when we speak of the colour of a person’s eye. 
Behind the pupil is the crystalline lens, E, having a much greater 
convexity at its posterior surface than at the anterior. 
The large posterior chamber is covered with the choroid coat, 1, 




