ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
117 
formed. It can be formed at any distance from the focal length of the 
objective np to infinity. 
In most Microscopes of the English and American model the tube is 
of such length that the image is formed at a distance of about ten inches, 
and that distance is therefore taken as the basis of calculation, and the 
formula then is 
1 _ 1 1 1 _ 1 1 
j p (object distance) p' (image distance) / (focal length) * 
or, substituting tho image distance 10 for p ' : 
1 
7 
With this formula let us work out the case of a 5-in. objective ; then 
1 _1 
p + TO ~ 5 
i a _i_ _ 
p ~ 5 “ lo “ 10 5 
p _ 10 
1 " 1 ; 
p = 1°; 
that is, the object and image are equidistant from the optical centre, and 
therefore of equal size, and there is no amplification. Of course, it can 
be assumed that the image distance p is greater than 10 in., as in case 
the draw-tube is used, when the formula will show that, with a 5-in. 
objective, the image is larger than the object, or p' can be taken as less 
than 10 in., when the formula will show that, with a 5-in. objective, the 
image is less than the object. 
Keeping 10 in. for our image distance, let us take the case of a 
1/5 in. objective, then / = - • 
o 
1 1 
P + To 
l 
l l 
i 
To 
50 
10 
10 
V ~ 49 
a (amplification) = 
10 
1 o 
4^ 
_ 49 
10 “ 10 
= 49 times. 
By this formula we can easily calculate the amplification of the real 
image projected at 10 in. by any simple-convex lens, if the focal length 
