312 Mr Barlow on the Practical Construction 
and the ratio of dispersion of the two .683 : also the required 
focal length 46 inches. 
To Jind the 'proper focal length of the two lenses forming the ob- 
ject-glass, so that they may have to each oilier the ratio of the 
dispersive powers, and a compound focal length of 10 inches. 
Rule. — Subtract the number, expressing the dispersive ratio 
from unity, and the remainder multiplied by 10 will be the fo- 
cal length of the plate-lens. 
2. Divide the focal length of the plate-lens so found by the 
dispersive ratio, and the quotient will be the focal length of the 
flint lens. 
Example . — In the case we have proposed the dispersive ratio 
is .683 : therefore. 
From 3.0000 
Take .683 
Remainder .317 
Multiply by 10 10 
3.17 inches focal length of plate. 
.683)3.170(4.64 inches ditto of flint. 
To find the first or exterior surface of the plate-lens, and the fourth 
or anterior surface of the flint-lens, for a compound focal 
length of 10 inches . 
We must here have recourse to the table given in the subse- 
quent pages, proceeding as follows : In the first column, con- 
taining all dispersive ratios, which ever fall within practical li- 
mits, find the particular one in question, as, for example, in our 
case .683 ; and in the same line in the second and fifth columns, 
will be found the proper radii of curvature for the first and 
fourth surfaces, provided the index of the plate be 1.524, and 
of the flint 1.585 : to which numbers the table is computed. In 
our case these numbers are 6.7956 and 12.7423. But when 
the tabular indices, as in this example, are not precisely those of 
the glass in hand, then the above tabular radii must be corrected 
as follows : 
For the Plate Lens .— Find the difference between the tabular 
index for the plate and that of the glass in question, and multi- 
