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fact each part which so intercepts one and transmits another por- 
tion of the ray acts as an origin of light; and that from every point 
of the object this light diverges in every direction as from an 
original source. Now, the microscope is designed to give vision 
by light thus divergent; all else is an intruder, and does harm 
by causing fog and glare, which tend to obscure by overpowering 
the erial picture. 
‘‘ That this divergence of light from the surface after transmission 
through the substance does exist, admits, I think, of no doubt. 
‘‘ Suppose a pencil of light passing through a dark space, and 
across the line of vision of a spectator, no impression whatever will 
be conveyed to his eye, save by reflection from particles floating in 
the atmosphere: let a translucent object of any kind be so placed 
that the pencil must pass through it, and immediately, no matter 
-what may be the obliquity of direction from the eye of the observer 
to the anterior surface of the object, it will become visible. 
‘Taking it then, as established that the only rays of light which 
assist in forming the erial picture on which microscopic vision de- 
pends are those which diverge from the object under observation, 
it follows that all other rays which enter the instrument, if they 
reach the eye, tend to confusion, and it becomes important to ascer- 
tain the best method of admitting such useless rays, when they can- 
not, or it is not desired that they should, be excluded. Without 
occupying time by discussing this point, I believe, as the result of 
consideration and experiment, that these rays ought to enter either 
so obliquely as to pass entirely across the axis, and thus not reach 
the eye at all, which is the condition of black ground illumination; 
or that they should be as nearly as possible parallel, in which case 
they come to a focus, close behind the object-glass, and, therefore, 
by their rapid divergence the greater part are thrown against the 
non-reflecting inside surface of the tube, and are thus absorbed, 
leaving but a small portion round the axis of the cone to reach the 
eye in a diffused state, giving a field more or less luminous accord- 
ing to the distance of their focus from the eye. 
“‘ Having satisfied myself of the truth of these premises, I sought 
for a construction which would realize them; first addressing 
myself to parallel light with a luminous field. 
‘* To obtain a parallel beam of condensed light is very difficult, 
