92 
Colouring of the Shells of Birds' Eggs. [January 
Since writing the above-named paper I have met with a 
number of fadts which make me more and more convinced 
of the truth of this supposition. I have found several 
excellent cases in which spedtra are closely alike in every 
particular except in the exadt wave-length of the centres of 
the bands, and though the two substances differ most 
materially in other characters, it is easy to prove that they 
are very closely related, and perhaps yield the same product 
when decomposed by the action of certain reagents. The 
difference of a few millionths of a millimetre in the wave- 
length of the bands, which a superficial observer might 
overlook, may thus serve to teach us far more important 
facts than if the spectra had been entirely different ; and is 
a good illustration of the importance of minute accuracy in 
scientific investigation, and of looking upon nothing as 
accidental and insignificant. 
A striking example of such a relation is the connection 
between the spectra of oorhodeine and of the product of the 
decomposition of the red colouring-matter of blood by strong 
sulphuric acid, discovered and described by Thudicum.* 
When in a nearly neutral state, dissolved in alcohol, it 
gives a spectrum of exactly the same character as that of 
oorhodeine in the same physical condition ; and on adding 
a small quantity of a strong acid to both they are both 
changed in the same manner, and give new spectra which 
are also of exactly the same character. The spectra of their 
neutral modifications and also those of the very acid 
solutions, have most remarkable and unu$ual peculiarities, 
quite unlike those of any other substances ; and therefore 
one cannot, I think, attribute the resemblance to mere 
accident. The agreement is so close that a superficial 
observer might easily be led to conclude that they were 
absolutely identical, and that oorhodeine was merely 
Thudicum’s cruentine ; but when the spedtra are compared 
together side by side with a suitable instrument, it may be 
seen that although the number, relative intensity, and 
relative position of the bands, both in a neutral and acid 
condition, are exactly the same, the position of the band is 
not the same. The difference between the spedtra is exactly 
like what is so often seen on comparing together the 
spedtra of the same substance dissolved in different liquids ; 
but this explanation will not apply in this case, because I 
find that the position of the bands in cruentine does not vary 
with the nature of the solvent, and the difference between 
* Tenth Report of the Medical Officer of the Privy Council, p. 227. 
