298 THE NORTHERN MICROSCOPIST, 
visible. It is at this spot also where the blood-vessels enter the 
eye, and ramify through nearly the whole of the surface layers of 
the retina. 
In the centre of the figure (25) you will see also a dark shaded 
portion practically free from blood-vessels. It is a round, yellowish 
elevated spot, about jth of an inch in diameter, and it is here 
that the sense of vision is most perfect. It is called the yellow 
spot of Scemmering ; it is not covered by the fibrous part of the 
retina, but a layer of closely set cells passes over it, and in its 
centre is a minute depression called the fovea centralis, 
In the short space of time at my disposal it has only been pos 
sible to give you a rough outline of the structure of the human 
eye. You must not think that I have exhausted the subject; I 
have only selected such matter as bore upon good or defective 
vision, or upon the construction of the mix roscope. Qn the other 
hand, I think enough has been advanced to show you that this 
organ is liable to imperfections which may, and are extremely 
liable to modify all our observations made over the tube. 
In order to produce a picture upon a screen, you will be aware 
that a lens is not absolutely necessary, if a diaphragm, perforated 
with a series of holes, be placed in front of the electric lamp, the 
screen will be decorated with as many images of the carbons as 
there were holes in the diaphragm; but another illustration will 
perhaps render this more evident. A small hole pierced in the 

Fig. 27. 
shutter of a darkened room (Fig. 27) allows of the passage of rays 
from a well illuminated landscape, so that a small but inverted 
image is cast upon the screen; the further the screen is placed 
away from the aperture the larger will the image be, though less 
distinct, and vice versa. ‘The picture produced is not so good as 
that formed by a lens, it is dark and somewhat confused at the 
margin, and if the aperture is enlarged, there is still greater con- 
fusion, until the image is finally lost. ’ 
Now, if we take an ordinary lens of glass and attempt to produce 
a picture with it, you will find the centre alone is plainly visible— 
the lens is afflicted with what is termed spherical aberration, that 

