1890.] Microscopy. 493 
filled with a thin layer of common olive oil, and is placed on a water- 
bath or in a small cupboard, such as are used for imbedding in paraf- 
fine, at a temperature of about 50° C. Under the influence of the 
higher temperature the oil gradually loses its yellow color and becomes 
thicker. The great point now is to select the right moment at which 
the oil will have attained the proper degree of thickness and viscosity, 
as also the other properties, which at present I am not able to define 
more exactly, but on which much of the success seems to depend. 
The exact moment can, however, only be found out by systematic 
trials. After the oil has been thickening for three or four days, a tria 
should be made with a drop of it in the manner described below 
Should the drop not become finely vesiculate, and exhibit little or no 
streaming, continue the heating process, and experiment again on the 
following day. If the oil should become too thick it will form good, 
frothy drops, but will scarcely show any streaming. In this case mix 
it with a small quantity of ordinary olive oil, and thus render it more 
liquid. If it has become much too thick it will form a good froth, but 
the latter dissolves very rapidly in glycerine. 
You see that the process to obtain the suitable oil is somewhat slow 
but I do not at present know of any other method by which the resule 
can be reached more quickly and surely. 
To prepare the vesiculate drops I proceed in the following way : — 
In a small agate mortar I grind a small quantity of pure dry carbonate 
of potash (K,CO,) to a fine powder. I then breath on the salt 
till it becomes slightly moist, and with a glass rod add to it a drop of 
oil, mixing the two constituents to a thickish paste. The success of the 
_ experiment depends, however, more upon the nature of the oil than - 
upon the proportion of the oil and salt in this mixture. Then, with a 
glass rod or a needle, I place a few drops of the paste, about the size of 
a pin’s head or smaller, on a cover glass, the corners of which are sup- 
ported by small pegs of soft paraffine. I then place on a slide a drop 
of water, and put the cover-glass over this in such a manner that the 
drops of paste are immersed in the water, but are not much compressed, 
to which end the corners of the cover-glass have been supported by the 
paraffine. The preparation is then placed in a deep chamber, and 
remains there about twenty-four hours. The preparation is then 
washed out with water by applying blotting-paper to one edge of the 
cover-glass, and supplying water at the other edge from a capillary tube. 
If the drops have turned out well, they will begin almost immediately 
after this to move about rapidly, and change their shape continuously. 
The water ons the cover-glass must now be displaced by glycerine, 
