118 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
of the lens be known ; and I present herewith a table so calculated for 
most of the focal lengths used for Microscope objectives. (Table A.) 
It will be noted that the amplifications obtained are, in every case, less 
than those obtained by the number of times the focal length is contained 
in 10 in., and the reason is that one conjugate focus (the image distance) 
being at less than infinite distance, the other conjugate focus (the object 
distance) must be at a greater distance than the focal length, and there- 
fore a quantity greater than the focal length must be used for the 
divisor, and the quotient (the amplification) must be less. 
As a 1-in. simple-convex lens amplifies the image projected by it at 
10 in. from its optical centre 9 times, a 1-in. objective should do the 
same (without reference to its actual focal length). If it fails to do so, 
if the image projected by it at 10 in. from its optical centre is amplified 
more or less than 9 times, then the objective has been incorrectly named ; 
it is not a 1-in. objective, but something else. 
The Ocular. — Having disposed of the real image projected by the 
objective, we come to the virtual image projected by the ocular ; here 
the formula is 
i _ 1 _ Jl. 
V v' - f ’ 
substituting the image distance 10 for^' we have 
1 _ 1 _ _ 1 
f 10 - /' 
With this formula let us work out the case of a 5-in. ocular : 
1 1 _ 1 
p “ 10 ” 5 
11 JL_ 3^ 
p ~ 5 + To “ lo 
10 
* = T 
10 30 
The wide difference of this result from that obtained for a lens of the 
same focal length used as an objective shows very plainly the absurdity 
of using, as many of us have done, and as many of the books teach, the 
same general rule for determining the amplifying power of objective 
and ocular, viz. to divide 10 in. by the focal length expressed in inches. 
I present herewith a table of amplifications of virtual images pro- 
jected at 10 in. by simple lenses corresponding to the most commonly 
used oculars. (Table B.) 
The total amplifying power of any combination of objective and 
ocular is obtained by multiplying the amplifying power of one by that 
of the other. 
