1076 
DR. H. WATNEY ON THE MINUTE ANATOMY OF THE THYMUS. 
blue. It is also better to keep the red solution very strong, and to filter a few drops 
of it into a watch-glass of water to the required strength each time it is used, as all 
filtering tends to turn the solution blue, especially if a clean dry filter-paper is used ; 
and the filtering of weak solutions is prejudicial to the red colour. The blue solution 
is used with more water, because the weaker the colour the bluer the specimen. 
Specimens of thymus treated in this way will, in most cases, show double staining; 
the difference of colour, it is true, is not much marked, but still is quite sufficient to 
clearly distinguish the cortical from the medullary part of the follicle (see Plate 83, 
figs. 1, 2 ; and Plate 84, fig. 3), or to stain the granular cells of the thymus, and 
the surrounding protoplasm, of two distinct colours (see Plate 85, fig. 8). 
The red hsematoxylin stains the connective-tissue, and the protoplasm of the con¬ 
nective-tissue-corpuscles and of the granular cells, and the walls of the vessels ; while 
the blue deeply stains mucus, almost all nuclei, the reticulum, and the lymphoid 
corpuscles. * 
As a rule, if only a single staining is used, the solutions which are of a purple colour 
are much the best; the red hsematoxylin solutions do not stain deeply enough, though 
specimens stained by them bear high powers well. The blue solutions leave the 
protoplasm of the cells almost uncoloured ; and the dark colour of the nuclei, and of 
the lymphoid cells, prevents the other features of the specimen from being well seen, 
in fact such specimens are very unsatisfactory. 
The difference in colour of the hsematoxylin solutions depends almost entirely on 
the alum -which is used in making them. It is true that the different woods give a 
slightly different extract, but the variation between the extracts, whether they are 
made from Campeachy or Jamaica, Honduras or St. Domingo woods, is so slight as 
not to be of any real importance. The alum by keeping becomes more acid, and 
acid, as is well known to chemists, turns the blue colour of hsematoxylin, red. The 
most intensely blue colour is obtained by using freshly-prepared dried alum, while old 
powdered crystals of alum (potash alum) give a most decided red or even a yellow 
colour. The proportion of alum to the hsematoxylin extract is also somewhat important. 
If the proportion sinks below three of alum to one of extract, the red colour will 
probably be seen ; but, in that case, the staining properties will not be so good. Any 
amount of alum, above three of alum to one of extract, appears to make little difference 
to the colour. To show the difference between the solutions when different alums are 
used, see Plate 86, fig. 22. The solutions whose colours are represented by a, b, d, e,f 
were all made with the same extract of hsematoxylin, on the same day, and with the 
same proportion of alum to extract, one of the latter to three of the former. 
* The words * £ lymphoid corpuscles ” are used in this paper in reference only to the smaller thymic 
corpuscles, such as are figured in Plate 92, figs. 72, 73, 74, 75, and 76, as The. 
