376 
Transactions of the Society . 
It has demonstrated the fact that, even with a narrow pencil of light 
from the condenser, we may be utilising the full aperture of the 
objective, for a single ray of light, in passing through the object, 
emerges not as one ray, but as several, forming together the so-called 
diffraction-fan, which is the more spread out the finer the structure. 
Moreover, we know that the innumerable narrow pencils of light of 
different obliquities which go to make up the illuminating cone, each 
form their own dioptric and diffraction pencils, and thus each elemen- 
tary ray incident upon any part of the finer structure, of an object 
utilises several zones of the objective at the same time.* 
Now besides, and combined with this polyzonal change of direction 
of an incident pencil, the direction of each incident pencil of light is 
more or less changed by the refractive and reflective properties of the 
object. 
Bearing this in mind, a fundamental difference in the formation 
of the image of the object and of the background becomes apparent. 
The object is seen by virtue of its action on the light in four 
ways, viz. by absorption, refraction, reflection, and diffraction.! It 
is obvious, therefore, that the direction of the rays of light incident 
upon an object must be changed to a great extent by the object 
itself. But the background of an object is seen solely by means of 
those rays which have passed the lenses without having had their 
direction changed by an intervening obstacle, viz. the object. 
It is this fact of which we take advantage in the method under 
consideration : we separate out as far as possible, and contrast the 
direct rays of light by which we see the background with those 
whose original directions have been changed, and by means of which 
we see the object. 
Ordinary dark-ground illumination affords a familiar instance 
illustrating the above general principles. Taking out the eye-piece of 
the Microscope and looking down the tube, no light whatever reaches 
the eye, the background appears black therefore. (See plate IX. 
fig. 3 and explanatory note on same.) When, however, some refracting 
object is placed in position, the field becomes filled with light ; this 
light depicts the object when the eye-piece is replaced. (See plate IX. 
fig. 4 and explanatory note.) 
Now, we can obtain dark-ground illumination in another way 
especially applicable to high powers. If we stop down the condenser 
cone of illumination until it only fills about a quarter of the back 
diameter of the objective, that is to say only ^ of the total area, and 
we then place a black stop behind the objective to cover this directly 
* Although it is obviously impossible to consider the action of a single ray apart 
from that of the pencil or beam of which it constitutes an element, I haye made use 
of this abstraction to lend greater clearness to the principle. 
f According to the nature of the object, the one or other of these four properties 
predominates. With very minute structures it is impossible, strictly speaking, to 
separate them, and I understand that in that case they are sometimes all classed 
under the head “ diffraction.” 
