12. When it is wished to fill the capillaries (the minute vessels connecting the arteries with the veins), the “Micrographic Dictionary” recommends the colouring matter to be made by double decomposition. As a professed handbook would be, perhaps, deemed incomplete without some directions as to the mode of getting these colours, I will here make use of those given in that work. For red, however, vermilion, as above stated, may be used; but it must be carefully examined by reflected light to see whether it be free from all colourless crystals or not. It must first be worked in a mortar, and then the whole thrown into a quantity of water and stirred about; after leaving it not longer than a quarter of a minute, the larger portions will settle to the bottom, and the liquid being poured off will contain the finer powder. This may then be dried slowly, or added to the size whilst wet in the manner before advised. 13. _Yellow injection._--To prepare this, take-- Acetate (sugar) of lead 380 grains. Bichromate of potash 152 ” Size 8 ounces. Dissolve the lead salt in the warm size, then add the bichromate of potash finely powdered. Some of the chromic acid remains free, and is wasted in this solution, so the following is given:-- Acetate of lead 190 grains. Chromate of potash (neutral) 100 ” Size 4 ounces. The first of these has the deepest colour, and is the most generally used. 14. _White injection._--This is a carbonate of lead:-- Acetate of lead 190 grains. Carbonate of potash 83 ” Size 4 ounces. Dissolve the acetate of lead in the warm size, and filter through flannel; dissolve the carbonate of potash in the smallest quantity of water, and add to the size: 143 grains of carbonate of soda may be substituted for the carbonate of potash. 15. For blue injection, which is not, however, much used with reflected light, as before stated, take-- Prussian blue 73 grains. Oxalic acid 73 ” Size 4 ounces. The oxalic acid is first finely powdered in a mortar, the Prussian blue and a little water added, and the whole then thoroughly mixed with the size. 16. It may here be repeated, that it is only when the capillaries are to be filled that there is any need to be at the trouble to prepare the colours by this double decomposition; and, indeed, colours ground so finely may be procured that the above instructions would have been omitted, had it not been supposed that some students might find a double pleasure in performing as much of the work as possible by their own unaided labours.