ISO 
Transactions of the Society. 
V. — Virtual Images and Initial Magnifying Tower. 
By E. M. Nelson, F.B.M.S. 
{Bead 1 Oth March , 1892.) 
Prof. Abbe takes exception to the manner in which microscopists 
deal with the magnifying power of lenses.* He there speaks of what 
is called the initial magnitying power of a lens, which is determined 
by dividing a conventional length of 10 in. by the principal focal 
length of the lens. 
Tims let m be the initial power, d the conventional accommodation 
or screen-distance of 10 in., and f the principal focal length of the 
lens, then m = - . 
Although this expression is convenient, an undoubtedly erroneous 
meaning is usually attached to it. Let me illustrate this by an 
example. Let f, the focus of the lens, be 1 in., and the accommoda- 
tion or screen distance d = 10 in., then by the formula m = 10. 
10 is then the initial power of the lens. It must be remembered 
that this initial power is altogether an abstract thing, and one which 
is not realized in practice, except in one definite case, which will be 
dealt with presently. 
It does not mean that if an object o (which we may call = 1) is 
placed before a converging lens of 1 in. focus, and a screen is placed 
10 in. on the other side of the lens, that i, the size of the image on 
the screen, will = 10. I will not deny that it has often been made 
to mean that ; it is nevertheless quite wrong to do so. 
1 must point out to those who are in the habit of making this 
serious mistake, that there are two points with regard to every lens 
which are of greater practical importance to the microscopist than the 
focus ; these points are p, the point where the object o is situated, and 
p' where its image i is formed ; p being the distance measured on 
the axis from the object o to the lens, and p' from the lens to the 
image i. 
Now f is the principal focus of the lens, and is the point to which 
parallel rays are converged, but as there are no such things as parallel 
rays in any part of a Microscope, f has no place in practical 
microscopy. 
The microscopist can therefore look upon / as an abstraction, and 
for him only useful for insertion in optical formulae. 
Now the point of first moment to the microscopist is the magnify- 
ing power of the instrument he is using, which in plain language is, 
how many limes larger than o does the instrument cause me to see i ? 
* Journal It. M.S., 1884, p. 348. 
