exhibited in its propagation along plates of glass, 
Tints. 
Values. 
Violet - 
7.20 
Indigo - - 
8.l8 
Blue - - - 
9.00 
Green 
97 1 
Yellow 
1040 
Orange - 
11.11 
Bright red 
11.83 
Scarlet - - 
12.67 
Now the difference of the values for violet and scarlet is 54 7, 
corresponding to seven different colours. Hence, upon the 
supposition that the eye can distinguish merely these separate 
colours, the accuracy of the scale is increased in the ratio of 
547 to 7, that is, from 1S7 to 239, which gives 3°.83 for the 
value of each unit. 
It is quite manifest, however, that we can distinguish at 
least three points in the developement of each colour; and 
even if this could not be accomplished by the unassisted eye, 
it can readily be effected, to a much greater extent, by cross- 
ing the fringe with a standard crystallized plate, and observing 
the degree of curvature which is produced in the fringes. 
This standard plate may be shaped like a wedge, so as to 
exhibit the variation of its tints to a great degree of minute- 
ness. In a wedge of this kind, two inches long, and ground 
out of a crystallized parallelepiped, so as to have an angle of 
8°, the highest tint is between the blue and the white of the first 
order, corresponding to 2.20 of the scale, and the lowest tint 
is between the black and the blue, corresponding to about 0.8. 
We have therefore a scale of nearly 2 inches to measure a 
variation in the tints amounting to 2.20 — 0.80 = 140, The 
