204. A. B. MACALLUM. 
structures coloured blue, obscuring the latter. This may be 
obviated, especially with high powers, by raising the Abbé 
condenser to the level of the stage and removing altogether 
its diaphragm, when the brilliancy of the light in the field of 
the microscope enhances the blue due to the iron reaction, 
while it renders more or less obscure the other details of the 
preparation. It was only in this way that I was able to deter- 
mine the occurrence of very minute traces of iron in the tissues 
and, when the sections were stained with safranin, of bodies 
which gave but a feeble Prussian blue reaction (figs. 45 
and 46). 
The sections of tissue were made, either by the free hand 
with a polished steel knife, or by the paraffin or celloidin 
methods. Care was taken that the knife should not yield a 
trace of iron to the sections. When the paraffin method was 
employed the surface of the cutting instrument was dry, but 
with the other methods it was covered with absolute alcohol. 
The transference of sections from one fluid to another was 
done with goose-quill points or with glass needles. 
I may not leave this part of the subject without a reference 
to the potash method for the liberation of ‘“‘ masked” iron, as 
described by Molisch, but afterwards determined by him to 
be untrustworthy. I have studied the effect of concentrated 
solutions of potassium hydrate upon vegetable tissues hardened 
in alcohol, and have obtained, frequently, evidences of the 
presence of iron in the cell wall, cytoplasm and nucleus, but 
the amount thus indicated in the last was always very much 
less than could be demonstrated by the other methods, while 
the reagent so altered the nuclei that a determination of the 
definite relations of the iron observed to the nuclear structures 
was impossible. My observations have convinced me that a 
very large part of the iron demonstrable after the use of this 
reagent is derived from the latter, however pure it may appa- 
rently be, and I am, therefore, upon this point in accord with 
Molisch. One of the readiest ways of proving this is by 
extracting all the iron from sections of vegetable tissues by 
keeping them in a quantity of warm Bunge’s fluid for several 
