_ ACCORDING TO THE UNDULATORY THEORY. 501 
When c is given it is easy to determine the difference 3 between the 
lengths of undulation for two minima lying close to one another, 
namely y and y — @. 
c 
If we then suppose oat 4 y and 
2 
c peat ahd); 
we have ee Tahoe Sait Re ie eg | 
For another length of undulation y/ we obtain in the same manner 
and consequently 
ENE fa il : ca - 
eae rey’ 
if ¢ is rather considerable, becomes very nearly equal to 
1 1 
e+y is she a 
and we then have also very nearly: VY =y%: y'2 . . . . (14) 
By means of which formula the comparison of the observed with the 
calculated phenomena may be effected. 
The locality in which I have hitherto performed my experiments has 
not allowed me to make an exact calculation of the fixed lines present 
in the solar spectrum, although this is the surest way to determine the 
relation between the refrangibility and the wave-length, because we can 
then make use of the exact calculations of Frauenhofer. I have there- 
fore only been able to decide as to the colour corresponding with the 
length of the undulation by the eye. The calculations then which I 
have hitherto made can only be considered as approximations ; Ishall not 
therefore produce them here. Notwithstanding, they have completely 
convinced me that the phenomena of absorption and those which must 
follow from the hypothesis laid down by me are identical. An example 
of this may, however, be worthy of mention, although the calculation 
must only be considered as an approximation. In the spectrum of iodic 
gas, fifteen stripes occupied 9' 30" from the orange-coloured to the red ; 
ten stripes between the yellow and green occupied 5! 30"; I therefore 
suppose that the distance between two neighbouring stripes, at the limit 
between the red and orange, amounts to 38", and at the limit between 
the yellow and greenjto 31". If we now insert in the formula (14), 
instead of y and y', the two corresponding wave-lengths (0-0000246 
and 00000219 English inch, according to Herschel’s table), we have 
d: d' = 38:30. 
As to the rest, it is self-evident that by the undulation-length of a colour 
I mean the length of undulation in the absorbing medium. Since inthe 
examples mentioned the lengths of undulation were taken as they were 
calculated in the air, and moreover the squares of the lengths of undu- 
2M2 
