Objectives for the Compound Microscope. 407 
the magnifying power, viz : 
— a and & =m, in which fis the length of the focus 
pp 
br parallel rays, p the distance of the lens from the object, p’ 
its distance from the image, and m the true magnifying power, 
that is, the size of the image divided by the size of the object; 
p and p’ are termed the conjugate foci and are variable quan 
tities; f is termed the principle focus, and has an unchangeable 
value for each single lens. : 
If now we combine the above equations, representing p + 
or the sum of the conjugute foci by /, we may deduce the formula 
i which represents in the case of any single convex 
lens the relationship existing between the length of the rincl- 
pal focus, the magnifying power, and the distance from the ob- 
is f 
ens m and / are given, the accuracy of the value of f resulting, 
will be limited only by the degree of precision with whic 
rated, in such a manner that the image of a micrometer should 
be focussed upon a white sereen, using of course no eye-piece, 
to measure the distance from the micrometer to the screen, to 
A » m : . 
many different values for fas there are distances , 108 
of obtaining but one value for all distances as we do with a 
Qgie lens, 
