238 Mr. Barlow on the rules and principles for 
tube considerably, that is to nearly 50 inches, we shall find 
by repeating the experiment, that is, by closing still more 
the two glasses, that the circular spot before the image is 
formed, and the surrounding atmosphere when the focus is 
obtained, will be red, orange or yellow; and between these 
extremes a focal length will be found where the circular 
spot in the middle will lose all distinguishing colour, showing 
itself a bright white ; and when in the focus the image will 
be colourless, although surrounded by a visible atmosphere, 
principally proceeding from a want of spherical correction. 
At this time the glasses are corrected for dispersion, and 
the compound focal length measured from the back of the 
flint, and the distance of the glasses must be accurately read 
off ; and with these data the dispersion may be obtained by the 
formulas already given, viz. : 
(f-*v 
ff 
- - - - (2) 
In this expression/ is the focal length of the plate lens for 
the given object, and f the focal length of the flint for parallel 
rays. The former may be found by direct observation with 
the index instrument, as already described, but the latter is 
best determined by means of the compound focus ; that is, 
calling the compound focus /", we shall have 
77=——^ ' ( 3 ); 
and /' being thus determined, is to be employed in the pre- 
ceding formula. 
As an example: suppose the compound focus measured 
from the flint to be 34 ' 89 inches, the focal length of the 
crown lens 9- 85 inches, and the distance between the lenses 
1-41 inches. 
