wm the Invisible Prismatic Spectrum. 177 
By means of measurements like the one described above, the 
deviations of various obscure rays of known wave-lengths were 
determined. The indices of refrastion were then computed by 
si d 
sntier® where a=62° 34’ 43”. The 
results are contained in the following table, where, however, 
only the final results of successful days are given, most of the 
observations having been lost through changes of the sky during 
the course of one determination.* 
the usual formula n= 
TasLe I.— Experimental determination of dor nas a function 
of A (Hilger prism). 
Date of observation. A ds n 
meee, F009 ot 1-910 +0°0053 46° 12” 15654 
EE RE ae 1:200 £00069 45° 54’ 15625 
ls) Saree 1°658 + 0°0091 45° 16’ 15562 
Were cae STi oe 1°767 40-0094 45° 087 15549 
ig BE aT 2:090 + 0°0104 44° 45’ 15511 
eet eG oey ik Sc 23564070110 44° 257 15478 
Spectrum alone, we can generally use with advantage a bolo- 
meter of as small an aperture us one-fifth of a millimeter, but 
that here it is advisable to open it to 2™" owing to the relative 
€xpansion of the spectrum and to the very feeble heat. 
wing to difficulties arising from the almost infinitesimal 
amount of heat in question, numerous subsidiary observations 
7re requisite for a single determination, which it therefore takes 
long to make, each final value resting upon between 20 and 
100 readings. If it should possibly appear to the reader birote 
in the three months of consecutive labor which were given to 
this part of the work, more than six points might have been 
ftermined in the curve, he is asked to remember that what is 
here difficult has till now been impossible. eee a3 
Plotting the points given by the data in table II and drawing 
* smooth curve through them, we obtain the curve of “ obser- 
“oe showing » as a function of A in the lower curve of 
ate \, : 
There would be no gain in accuracy, at this stage, In at- 
tempting to work from a formula representing 
trustworthy as the data. This I say with special reference to 
* All these observati i i i een n and 7, can be 
ervations, for discovering the relation betw: — 
fonducted with at | much advantage by a powerful and constant electric 
light at least as 
Gana by sunlight. The latter only, however, was at the obser 
the equation of > 
the curve obtained, as the graphical construction is fully as : 
ver’s actual com- 
