180 PREPARATION AND MOUNTING 



15. For blue injection, which is not, however, much used? 

 with reflected light, as before stated, take 



Prussian blue 73 grains. 



Oxalic acid 73 n 



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 plea- 

 sure in performing as much of the work as possible by their 

 own unaided labours. 



17. The process of injection may now be considered ; but 

 it is impossible for written instructions to supply the place 

 of experience. I will do my best, however, to set the novice 

 at least in the right way. There are two kinds of injection 

 one where the object and colours are opaque, and conse- 

 quently fit for examination by reflected light only; the 

 other, where the vessels are filled with transparent colours, 

 and must be viewed by transmitted light. The first of these 

 is most frequently employed, so we will begin with it. In 

 the object which is to be injected, a vessel of the kind which 

 we wish to be filled must be found ; an opening must then 

 be made in it to allow one of the small pipes before men- 

 tioned to be thrust some distance within it. When this is 

 accomplished, thread the curved needle with a piece of silk 

 thread, or very fine string, which some operators rub well 

 with beeswax. This thread must not be too thin, else there 

 is danger of cutting the vessel. The cord is then carried 

 under the inserted pipe, and the vessel bound tightly upon 

 it, the ends being brought up round the transverse arms, 



