in the Invisible Prismatic Spectrum. 171 
heat is on the average less than one-tenth that from the prism. 
We must use too, if possible, a narrow aperture to register this 
heat; for a broad one might (on account of the compression of 
the infra-red by the prism), cover the whole field in which its 
work should be to discriminate. (2d.) We must have not only 
an instrument more sensitive than the common thermopile, but 
we must devise some way of fixing, with an approximate pre- 
cision, the point at which we are measuring, when that point is 
actually invisible. | 
he apparatus I have devised for this double purpose, has 
done its work with a degree of accuracy, which if it may be 
given e perimental data very far outside the visible spectrum, 
by which we may either construct an empirical formula and 
Supply its proper constants so that it will be trustworth y within 
extended limits, or test the exactness of such formule as 
Cauchy’s, Redtenbacher’s, ete., which, while professing a theo- 
retical basis, only agree in their results within the limits of the 
Visible spectrum (from which they have been in fact derived, 
and where they are comparatively unneeded). They contra- 
dict each other, as will be seen, as soon as they are called on 
for information in the region outside of it, where they would 
chiefly useful. oe 
the present work has been preceded by a new map of the 
may be hoped that these experiments will be 
Prisms of other material, and by other observers, now that th 
Preliminary obstacles have been remo 
] ved. bees nasi 
In order to map the spectrum on the normal scale, where i | 
si 
