386 Transactions of the Society. 
tapering tip (fig. 5, d), for needles with a long taper are apt to 
break anywhere. 
A necessary precaution is to have the capillary from which the 
needle is to be made perfectly dry. The presence of the least 
moisture may result in alternating columns of water and air in 
the pipette tip which no amount of pressure will expel. 
Water seems to be the best medium for transmitting pressure 
in the apparatus. Mercury is apt to break and allow air or water 
to leak past it when it reaches the tip of the pipette. When this 
occurs the separated droplet of mercury clogs the aperture. 
Mercury also tends to leak past the best plunger made.* The 
disadvantage of using water is the risk of its diffusion into the 
solution to be injected. If a considerable amount of the solution 
be drawn into the pipette this risk is minimized. A good method 
is to colour the water (e.g. with Nile blue chlorhydrate or with 
neutral red). The solution drawn into the pipette from a hanging 
drop is then visible by contrast. For ordinary purposes a cushion 
of air between the water and the injection fluid serves well. 
Oil is unsuitable because, in spite of all precautions, it 
occasionally comes into contact with the hanging drop containing 
the tissue to be operated upon ; it then spreads over the surface 
of the drop and injures the preparation. It also dissolves de 
Khotinsky cement and sealing-wax, which are so convenient for 
cementing the pipette to the apparatus. 
Manipulation of the syringe is facilitated by fastening it in a 
frame, and by using a milled screw to press the plunger. I use a 
microscope for this purpose with the objectives, substage, and 
mirror removed. The syringe is passed through the centre of the 
microscope stage, where it is held firmly with a tight-fitting collar 
of cork. The lower end of the microscope tube rests on the top of 
the plunger, so that pressure can be brought to bear on it by either 
the coarse or fine adjustments. There is no need of fastening the 
plunger to the microscope tube, because the resiliency of the water 
in the apparatus is sufficient to cause suction in the micro-pipette 
when the plunger is released from pressure. 
Appendix. 
Barber’s instrument is based on the principle of a carrier 
pushed along a groove by a screw at one end. By having a series 
of three carriers built up on one another, each travelling in a 
different direction, movements in any one of three dimensions may 
be imparted to a needle clamped to the top carrier. Hecker 
(1916) improved Barber’s instrument, but added materially to the 
intricacy of its manufacture. 
* Leakage in, the syringe can be avoided by placing a cushion of oil between 
the plunger and the mercury. This may also be done when water is used. 
