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distance of their principal focus. The object to be viewed is, of course, 
to be placed where these axes intersect, and the head raised so far above 
the instrument that the eyes, when directed towards the object, may 
look through the centres of the two lenses. This will require the eyes 
to be raised about three inches above the lenses. The object will be 
then seen standing out in the strongest stereoscopic relief, showing many 
details which cannot otherwise be seen with the same power; and en- 
abling dissections to be made with the greatest ease. 
NN 
TL Ain eS 
The inclination of the axes of the lenses should not exceed an angle 
of about 25°, which requires, if the lenses be of large aperture, that 
a small segment of each be ground away where they are in contact. 
Another essential condition is that the line in which the planes of the 
lenses intersect be perpendicular to the line joining their centres. The 
degree to which this has been attained may be readily tested by viewing 
a window-bar by reflection in the mounted lenses; and if the adjustment 
prove defective, it may be rendered complete by paring one of the cells 
in which the lenses lie. A pencil line should be drawn down the middle 
of the board or box which forms the foot of the in strument, to mark where 
the object is to be placed. 
There are two advantages in having the lenses mounted so close to- 
gether that the eyes need to be withdrawn to some distance in using 
them. It prevents any sensible squint, and it enables parts of the 
object much nearer than the focus of the lenses to be seen distinctly, 
simultaneously with the more remote parts. The result is, that the 
stereoscopic effect is complete, and that the feeling of perfect tranquil- 
lity in the eyes enables the instrument to be used for any length of time 
without a sense of strain. 
While working with an instrument of this kind there is a tendency 
from natural associations to bring the eyes gradually too close. This 
should be checked from the first, and the habit will soon be conquered, 
