determining the dispersive ratio of glass, &c. 
^36 
Method of determining the dispersive ratio.^ 
3. The instrument employed for this purpose is similar to 
that used for determining the index, except that the tube, 
instead of being only ten inches in length, consists of three 
joints, one 20 inches, and two others 10 inches each, the 
draw tube being about 14 inches long (graduated as before); 
so that the length may be conveniently varied between 20 
and 50 inches. The cell, which carries one of the lenses at 
the extremity of this tube, screws inside flush with the tube 
itself, and will thus admit another tube about 20 inches long 
to slide over it; at the extremity of the latter is another cell 
for carrying the plate lens. 
This exterior tube is moved over the other by means of a 
tangent screw and handle, with Hook’s universal joint, as in 
the adjustment of transit and other instruments. Moreover, 
the exterior tube being opened for the space of two inches, 
and the interior tube graduated, the distance of the two 
lenses from each other may always be read off to the hun¬ 
dredth of an inch. 
The instrument being thus described, the method of using 
it, and the principle on which the determination rests, will be 
readily understood. It is well known that in order to produce 
* I am not aware that this very sim^ile method of determining dispersive ratios 
has been before practically employed; but it is suggested by Mr. Herschel in 
his recent article refered to in a former page. He deduces it from considerations 
relative to the achromaticity, when the two lenses of an object-glass are placed at 
a distance from each other ; his primary object being to complete any trifling want 
of correction by a change of distance. My views were not very dissimilar, and 
our resulting equations, although differently expressed, are of course equivalent. 
