604 Price. — Some Studies on the Structure of the Plant Celt 
those who have taken an interest in the work and often helped with dis- 
cussion and hints I desire to express my best thanks, and also to Mr. F. F. 
Blackman, who kindly read the manuscript. 
Observations have chiefly been made with a simple type of dark- 
ground illuminator — the Zeiss paraboloid (Zeiss pamphlets) — used in con- 
junction with various lenses. Although dry lenses are recommended, the 
best results have usually been obtained with the 3 mm. Zeiss immersion 
apochromat and suitable stop. A clear image with few diffraction colour 
fringes is obtained by use of this, chiefly with the oculars 6 and 8. As 
illuminators, a Nernst lamp, sunlight, and a small arc lamp have been used. 
The last of these, a semi-automatic pattern, is most satisfactory for use in 
most cases, although the intense brilliancy of sunlight renders visible even 
smaller particles. The light is concentrated on to the plane mirror of the 
microscope by means of a spherical glass flask of water, which serves to 
absorb much of the heat and prevent undue heating of the object on the 
stage. 
Object-slides and cover-glasses must be flawless and of good quality ; 
they are specially cleaned and kept in alcohol. The object, mounted in 
water in the ordinary way, is first viewed with a low power and the 
illumination is adjusted, the final refinement under the high power being 
best performed by trial. In this way certain structures of the cell can often 
be more clearly brought out. 
Considerable difficulty is experienced in selecting material suitable for 
observation and experiments in various directions. Gaidukov showed that 
only in certain cases was good observation possible. The considerations 
affecting the suitability of material are roughly as follows : the tissues or 
structures must generally be only one cell in thickness ; the walls of the 
cells must be optically homogeneous ; the diameter of the cell or cell- 
filament must not be so small that the diffraction image of the walls inter- 
feres with observation of the cell contents ; and the cells must not contain 
a large number of bodies of the nature of chromoplasts, as these, by scattering 
the light, interfere with good observation of the more minute structures. 
All these criteria must be taken into account, and it may be seen at once 
that the field of choice is comparatively limited. This is obviously a barrier 
to the extended use of the method, and one which in many cases must 
apparently prove insurmountable. 
Only a small quantity of material must be taken — a few filaments of 
Algae, a single small leaf, and so on — and this is mounted so as carefully to 
exclude air bubbles. 
In experiments dealing with the action of reagents in the cell, in most 
cases considerable difficulty has been experienced, chiefly as to the possibility 
of observing the first action of the reagent. Two methods were adopted ; 
in that generally employed, the solution was added to the edge of the 
