THE ILLUMINATION OF MICROSCOPIC OBJECTS. 37 
powerfully on the plate as the light which passed the sides of 
it. Nowa purple object absorbs green light from the middle 
of the spectrum and transmits red and blue from the ends, so 
that if a green filter be introduced between the bull’s eye and 
the condenser the only light which falls upon the object is of 
the colour to which it is opaque. The green light passing the 
object affects the sensitive panchromatic plate, but that which 
falls upon the object is completely stopped and the plate is 
unacted upon in way of the image, which appears on the 
developed negative as almost clear glass. An example of this 
is shown in fig. 2, which represents a longitudinal section of 
Chara fragilis which has been stained faintly with gentian 
violet and photographed with green light. This method 
lends itself to the resolution of detail, which is sometimes 
invisible by direct observation. In fig. 3 is reproduced a 
photo-micrograph of a transverse section of Clematis vitalba 
doubly stained and taken upon an ordinary plate, while in 
fig. 4 is the same object photographed on a panchromatic 
plate with a green filter interposed. Thesclerenchyma, which 
is quite invisible in the first photograph and could scarcely 
be detected by ordinary illumination in the microscope, is 
very clearly defined in the second photograph, By photo- 
graphing a coloured object with light of the colour which 
it absorbs any desired amount of contrast can be obtained. 
This, of course, can be overdone, and excessive contrast 
obtained at the loss of detail. For. objects possessing con- 
trast in themselves it is better to photograph with the colour 
which the object transmits, as this brings out detail to its 
fullest advantage. Like all other processes this must be used 
with discretion, but it is easy to try one or two filters and 
compare the results, when the best can be selected. When 
these points are attended to, the definition should be suff- 
ciently sharp to show the minutest details, and even the 
marks sometimes left by the microtome knife in cutting the 
sections may be rendered on the plate. An example of this 
may be seen in fig. 5, which shows a section of the lichen 
Stereocaulon coralloides under critical illumination. 
