and the Achromatism of Microscopes. 2937 
This may be made the same for rays of all colours, or all may 
n 
—1 
: é é 
be made to converge to the same point, if Wl be always 
= 6 = for then by making 
iY tg 
jase dah or f=cF, 
the power of the compound lens for rays of all colours is 7 a aha 
fi 
is found in practice that can be very nearly, but not exactly, 
n—1 
én 
ke 
SIT for rays of all colours, ¢ being nearly =1, 5. 
expressed by c. 
Object-glasses therefore may be made on this plan very nearly, 
but not quite, achromatic. 
(4) Instead of placing the lenses in contact, suppose them to 
be separated; let their distance = a. The rays which fall on 
the second lens are converging to a point whose distance is F- a: 
hence the power after refraction at the second lens is 
1 
1 F 1 
F-a f a Cale 
F 
for mean rays. For rays of any other colour, it is 
F F'n-1 ai n-1 
or, by expanding and neglecting eS) &ec., the power is 
is. F én 
1 
F-a yo Woe tn—1  f wat 
HH 2 
