
302 THE NORTHERN MICROSCOPIST. 
every one knows how delusive an inexperienced estimate of this is. 
At sea, a landsman could not judge of the distance of a passing 
vessel to a few miles, nor could we form any accurate idea of the 
size of any object emitting practically parallel rays without we had 
something to compare it with. 
We now come to a point which has been much disputed in the 
study of microscopy—binocular vision. 
The two eyes move together as a system, so that we direct the 
two lines of regard to the same point in space and consequently 
see but a single image ; but it is possible to see two—if one eye be 
displaced a little with the finger two images are seen, while if the 
other be displaced to a corresponding amount the one image is 
restored, 
The value of binocular vision may be easily ascertained by experi- 
ment. When a picture is presented to the retina of each eye, the 
compound picture is much brighter than when one retina only is 
employed. 
To each point of the retina of one eye there is a corresponding 
point in the retina of the other, and impressions produced on one 
of these points are in ordinary circumstances indistinguishable from 
a similar impression produced on the other. 
When both retin are similarly impressed, the general effect is 
that the impressions are more intense than when one eye only is 
employed ; and we also get a perception of relief, that is of form 
in its three dimensions. 
Take two A eyepieces and look through them to the sky, so that 
two distinct circles are seen ; now bring them pe, pa so that one 
circle overlaps the other, when this overlapping bi-convex portion 
will be found double the brightness of the remaining portions of 
the circles, 
We are indebted to stereoscopic vision for the perception of 
relief ye ey in tee dimensions, which occurs when the 
images falling upon the correspondi ints of the two retin are 
not siinctty slender In looking aoe bien with both eyes the 
rays do not run parallel from one side of the object to the eye on 
that side, but the right eye centres itself to the left side of the 
object and vice-versa. This may readily be seen by holding up a 
finger between our eyes and the wall, and looking at the latter. 
Two fingers may be seen projected on the wall, one of these is seen 
by the right eye and the other by the left; but our visual impressions 
do not inform us which picture is formed by which particular eye. 
Now, while steadfastly looking at the wall, close the right eye and 
the left finger will disappear, while on shutting the left eye, the 
right finger is rendered invisible. 
_ When two similar pictures are presented to the eyes, the impres- 
Sion is more vigorous and looked at with greater ease than when one 
eye only is employed ; vision in this case is called pseudoscopic. 

