s6 
jured to identify them when examined with the prism at a certain 
distance ; but this varies with the refractive power of the prism, 
wherefore some index is necessary to determine it. 
I considered that, as perfect red is equal in breadth to the 4th part 
of its own productor, and is equal to the pale blue above any pro- 
ductor, so an increase of width at one end of the wedge, of one fourth 
part, will be equal to the width of the perfectest red or pale blue to 
be expected from it, and it will be a regulator or index for the whole 
of the instrument. 
Thus when the person has the prism so placed, that the upper 
edge of the light blue, above the first wedge, is in a right line from the 
edge of this index, or the dark blue upon it, I expect a similar 
breadth of scarlet will be at the bottom, adjoining an equal breadth 
of red under the index, supposing the index formed at the top, for 
it may be either at the top or bottom, but I prefer the top as being 
least in the way, (see the left hand end of the upjier figure, Tab. 7.) 
This wedge, at the beginning including the index, contains five, 
divisions which is equal to a much broader black space, for perfect 
red is given where it is four parts, and no greater width will give so 
perfect a red, but will always give scarlet. It will be seen that this 
proportion is observed in the examples of T«Z'. 3. and 4. In achro- 
matometer it should not be forgotten, that as few lines as possible 
should come in the way, and that at any rate extra horizontal lines, 
from the construction which at present seems best, will be greatly 
in the way, and even perpendicular ones will be a little so if not 
drawn quite through the scheme ; but to avoid both, I have managed 
that the productors in the lower figure shall so correspond with a 
certain measure that it may be immediately understood. But to 
proceed with a sketch of the upper wedge. As an inch and :|:th pro- 
duces scarlet, when viewed at a foot or more, one inch produces red^ 
and Aths produce lighter red approaching crimson : thus we may 
pass from the upper portion of the wedge to the next, beginning im- 
mediately where the above left oft', and consequently with the same 
