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determining the dispersive ratio of glass ^ &c. 
equally convex in the plate, and piano concave in the flint; 
their focal lengths varying in the plate and crown from 9^ 
to lo^- inches, according to their respective refractive indices, 
and in the flint from about 10 to 11^ inches. 
4. The flint lens, as will have been observed from the pre¬ 
ceding description, is placed in the interior tube, and the 
plate in the exterior; and if when the two interior faces of 
the lenses are in contact, the index does not read zero, its 
actual reading is recorded; and ultimately, the index reading 
when the image is colourless is corrected by this quantity.* 
This preliminary being attended to, the manner of con¬ 
ducting the experiments is as follows. 
Fix up the black board with the circular white spot, as 
already described, at a convenient distance, and in a good 
light, directly opposite the tube properly mounted on its 
stand. 
Let the two lenses be placed nearly in contact, and suppose 
the length of the tube reduced to about 20 inches. Now, 
move the plate lens gently forward by means of the handle 
and screw, the eye being placed at the eye-glass, and the 
image of the circular spot will, after a time, begin to appear 
in the field of the telescope, having a bright and strong violet 
spot in the middle ; at this time a very little farther motion 
in the plate lens will give a distinct image of the object, but 
encircled by a strong violet shade. 
If now the tube be lengthened to about 25 inches, and the 
experiment repeated by closing the glasses, the violet spot 
in the middle, and the circular ring when the focus is obtained, 
will have changed to a fine blue. If again we lengthen the 
• The different thicknesses of the lenses render the correction necessary. 
