114 PHOTO-MICROGRAPHS. 
purpose of obtaining a fluid of still higher refrac- 
tive index, would in certain cases give still better 
results. 
With a homogeneous immersion achromatic 
condenser, a homogeneous immersion objective, 
and a mounting medium having the refractive 
index of the glass slide and cover, it would seem 
as if the optical conditions should be as near 
perfect as possible. 
There is ample room for experiment and im- 
provement of the technique, and the writer can 
commend this fascinating art to those who have 
patience and are fond of overcoming difficulties, 
as one well worthy of occupying their leisure time. 
Moreover, the knowledge of histology gained in 
searching for suitable objects to photograph will 
be of a practical kind, like the knowledge of 
anatomy gained in the dissecting-room, and the 
time expended in this way will not be lost. 
The student of vegetable histology is advised to 
make sections showing the bark, wood, and pith 
of exogenous stems (see Fig. 1, Plate XI); the 
fibro-vascular bundles in endogenous stems (see 
Fig. 2, Plate XI., and Fig. 1, Plate X.); spiral 
vessels, annular and pitted ducts, scalariform ducts 
(in ferns), etc. He is also advised to make photo- 
micrographs of the various kinds of starch i situ 
in the cells of succulent shoots, roots, and tubers, 
—to be mounted in glycerine,— and of raphides 
(crystals found in the cells of certain plants). 
Cross sections of the stem of “ Wandering Jew” 
(Tradescantia) will show very beautifully small 
