The Illumination of Microscope Objects. 
405 
opinion is based upon experiments with a novel method of illumin- 
ation largely employed by Sir Herbert Jackson in work on the 
examination and identification of small particles in various media, 
notably in glasses and glazes, but he has used it also for the study 
of a number of other objects, including diatoms. In principle it 
consists in illuminating the object with plane polarized light, using 
the full angle of the dry or, in many cases better still, an immer- 
sion condenser, and examining the object with the analyser over 
the eye-piece in the crossed position. The appearance of the field 
is similar to that obtained with the black-ground illumination, in 
that the -small elements in the objects show up bright on a rela- 
tively dark ground. The plane polarized light meets the surfaces 
of the discontinuities in the object at various angles. It is con- 
verted into elliptically polarized light by reflection, and a portion 
of this elliptically polarized light is then transmitted through the 
analyser. Each reflecting point in the object is illuminating the 
portions round about it with this elliptically polarized light, and a 
completely illuminated object is seen. The light so reflected is 
somewhat faint, and a powerful source is essential. 
In this method, as with dark -ground illumination, whatever 
resolution is eventually obtained is seen immediately, whereas 
with transmitted light ultimate resolution is only obtained with 
infinite care in the manipulation of the light. The explanation is 
that there is no glare or flooding, while with transmitted light it 
is probably never entirely eliminated. 
Sir Herbert Jackson’s method is of such interest that it should 
receive the careful attention of microscopists. 
Mr. Akehurstin July, 1921, put a note in the English Mechanic 
in which he pointed out that certain animalculse and bacilli showed 
structure under polarized light, and it was probably due to the 
same cause. 
