f, /,v u.r. 
2 
V* II f, 5 v I KV 
; r A £, V i i 
COLOUR-REACTION. 
staining, when either the contrasts or the blending of the 
colours are so effective, but which assist him in tracing the 
continuation of a given tissue or the course of a vascular 
bundle, or reveal the structure of the otherwise too transparent 
tissue. The advantages of mounting serial sections are also 
much enhanced by the delicate shades of colour produced by 
a careful use of double stains. Of course for mounting, the 
colours produced require to be permanent, but transient 
colours may be equally valuable as a test to the biologist. 
Unstained sections are always the truest to Nature, and in 
many instances are sufficient for the biologist; but if no 
stains had been used, our present advance in the knowledge 
of the structure of the tissues composing living matter could 
never have been attained ; as sometimes even a cell-wall will 
be so delicate and transparent that it requires the most careful 
treatment to reveal its presence. 
Single stains are rendered valuable, as they colour some 
portions only cf the structure while the other parts remain 
uncoloured ; for instance, if the leaf of a Fern, Pteris tremula, 
after being “bleached,” were submitted to the action of a 
carmine dye, the beautifully reticulated epidermis would 
remain unchanged, while the sori, spiral springs, and spores 
would all be stained carmine, and so at once easily recognised. 
The foundation of double staining is that it is found from 
experiment that different colours affect different portions of 
the section, so that, for example, carmine and light green, 
being complementary to each other, please the eye ; while 
each colour reveals its own portion of the structure. 
Other double stains are needed, as some substances do not 
take the ordinary dyes, i.e., a section of the Lichen, Lecidea 
sctnyuinaria, while unaffected by the usual dyes, may have its 
fruit (apothecia) coloured by an aqueous solution of Morrell’s 
coral-red ink, and the remainder by an alcoholic solution of 
methyl green, giving brilliant results. 
Besides the large number of useful aniline dyes now in 
the market, sections may be stained by many special methods 
which the worker will have to find out for himself. 
Sections treated with an iron solution first, then by cyanide 
of potassium, will develope a fine Prussian blue. They may 
also be coloured brown by soaking them in a solution of 
nitrate of silver, and, after the mounting is complete, by 
exposing them to the sun, when the silver will turn brown, 
and so reveal the portions it has penetrated. 
It must be noted that in staining it is requisite to make 
the sections sufficiently dark to stand the subsequent process 
of mounting, but if too dark the delicate contrasts will be lost. 
