AMERICAN JOURNAL OF SCIENCE. 
, 
[THIRD SERIES.] 
Art. XIV.—On the Color Correction of double Objectives ; by 
CHARLES S. HASTINGs. 
I. Review of attempted methods. 
IF we call the focal length of two thin lenses in contact P, 
one of the lenses being made of a material with indices of 
refraction n, and the other with indices indicated by n’, we 
may write 
papain) A +(n’—1) B, (1) 
Where A equals the sum of the curvatures of the two surfaces 
bounding one lens and B the corresponding sum for the other. 
In order that this system may be achromatic it is necessary 
that P or ¢ should be invariable with n and n’, or in other 
words that the equations 
ae 
dn' 
A=—7- B (3) 
should be satisfied. Unfortunately this is in practice impossi- 
ble, because at present no two varieties of glass are known in 
which Fa constant. On the contrary, this coefficient is itself 
Am. Jour. sas Senet Series, Vo. XXIII, No, 135.—Manou, 1882. 
