438 Mr. M. Ponton on the Law of the Wave-lengths 
and to 860° add 200, being the number of degrees in the are 
AM, so making 560; then dividing, as before, by 720, there 
results 0°77! as the fraction corresponding to the mean wave in 
the foregoing series, which must accordingly be divided by this 
fraction. The quotients, taken in their reverse order, stand 
thus: 12857, 1/1428, 1:0714, 0°9643, 0°8571, 0°7714, 0°7282, 
0°6429, the mean wave being =1. 
The relative wave-lengths, however, as determined by Newton, 
are represented, not by these numbers themselves, but by the 
cube roots of their squares. The series then becomes 1°1824, 
10931, 1:0470, 0°9761, 0-9023, 0°8411, 0°8057, 0°7449, the 
length of the mean wave being =1. The ascertained length of 
this last in air is 0°00002247 decimal parts of an English inch ; 
and the lengths of the others are found by multiplying by this 
quantity the above numbers; so producing the following series, 
which represents, according to Newton’s estimate, the extreme 
wave-lengths between which the different tints of the solar spec- 
trum lie, in decimal parts of an English inch :— 
Red 0:00002657 
Orange 0:000024.56 
by allay 0:00002353 
Gen 0:00002193 
Bing 0:00002028 
Indigo 0:00001890 
| Pes 0:00001812 
0:00001674: 
This accordingly is the estimate given of these wave-lengths 
in all the English works on physical optics. , 
The next step in this inquiry was made by Fraunhofer, when 
he discovered the fixed lines of the solar spectrum, and deter- 
mined the wave-lengths corresponding to the points occupied by 
the seven principal lines, which he designated by the letters 
B, C, D, HE, F, G; H. His observations are understood to lave 
been made with most careful accuracy; and he left on record 
two sets of values of these wave-lengths, agreeing as respects B 
and D, but differimg slightly as respects the other five. These 
two sets of values, stated in decimal parts of a French inch, the 
standard employed by Fraunhofer, are as follow :— 
I. B0-00002541, €0-00002425, D0-00002175, E0-00001943, F0-00001789, 
II. B 0-00002541, C 0-00002422, D0-00002175, E0-00001945, F0-00001794, 
8 
I. G0-00001585, H0-00001451 
II. G0:00001587, H0:00001464 
No attempt has yet been successfully made to reduce these 
