and the Achromatism of Microscopes. 239 
This will be the same for all colours, if 
a ea e 
pig iigt pr 
which will determine /’ and” from /. 
(7) This is the principle of Dr. Blair’s object-glass. A_ re- 
fracting fluid (a solution of some metal in muriatic acid) is inclosed 
between two lenses of crown and flint-glass. It is found im 
practice that object-glasses thus constructed do not last, owing 
to the chemical changes in the fluid, and its corrosive action on 
the glass. 
(S) The separation of the lenses promises, in theory at least, 
to produce beneficial effects. Suppose a convex crown-glass lens 
whose focal length is F, and a concave flint-glass lens of focal 
length f# to be separated to the distance a: then (see 4) the power 
of the combination is 
1+ on 
n—1 1 én’ 
Fe ie 1+ 7 ) 
ras én 7¢ n'—1 
n—1 
oe r. F = (—% “)' si de» pr n' 
- F—a_ (F—a)?'n + oy ae : i ea 
Putting f aa th ion c an +e ee. this combi 
utting for wai e express 7 ease s - 
nation is achromatic, if 
i oa LAT erg 
(F—a)’ f (F- a) : 
From these equations a= ——-F. The power of 
e€ 
cae fe Tee (c+e) 
the compound lens 
1 
Pay age ie e 
