232 Transactions of the Royal Microscopical Society. 
which to a degree is evident in the blue specimen, is entirely 
wanting in the red. I annex the ratios of Mr. Sorby’s two read- 
ings and my own, to show how nearly they agree, and Mr. Sorby 
has since told me that his specimens were dissolved in a strong 
solution of sugar, which quite accounts for the slight displacement 
I have recorded. 
Fic. 7.—Rep Crneraria. 
) 
M. A. | Observations. 
| 
1 | 22-95 | Centre =. «. 585° | Class 1. Ends shaded, centre dark; 
size *5. 
2 | 21°86 + aac 541° », 1. Same as No. 1; size *465. 
Bi e2029> 45 a ee 499° », 1. Shaded evenly ; size -455. 
Condition, A. Ae} |r A. Ratio. 
Blue Cineraria.. 615: 575° 535° 490°1 9265 
Red ¥, es 585° 541° 499° a 1 3. -9234 
55 59 % 594° 550° 509° is £29258 
Lobelia speciosa 619° 573° 529° DS 29257 
There may, I think, be found in this alteration of colour some 
similarity to the chemical effect caused in litmus, when that sub- 
stance has, through the intervention of an acid or alkali, been 
turned red or blue, as the case may be; and perhaps I may be per- 
mitted to add that I have employed the spectroscope in many 
cases where the determination of an acid or alkaline base has 
been necessary in chemical analysis. A glance at Figs. 8, 9, 
and 10 will suffice to show what takes place when the litmus is no 
more alkaline or acid, as the case may be: the moment at which the 
change occurs is the saturating point, and it is at times extremely 
curious to observe the sort of conflict which takes place between 
the two agents, till,.when subsidence occurs, we have either a 
normal, acid, or alkaline result ; the only point necessary is, that the 
original solution of litmus be made of a definite strength, and must 
naturally represent the normal state; that employed in these ex- 
periments is of 1 per cent. 
Now this seems to imply that the red colour of a fresh plant is 
more especially acid than the blue ; still the conditions of variation 
are so exceedingly numerous, that to fix anything like a definite 
why or wherefore as to the cause is quite out of the question, at 
least so far as my own short experience of nature and natural 
selection is concerned, I therefore must be pardoned if I shade 
myself under the following extract, taken from Mr. Darwin's 
admirable work, entitled ‘The Origin of Species.’ He there says: 
“ When a variation is of the slightest use to any being, we cannot 
