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beam, which at present I incline to believe should just fill the 
field of view, especially for objectives of such large aperture as 
are now in use; as these, if the illuminating spot be larger than the 
field, collect too much of the light, which it is the object of the 
construction to get rid of. Whether this may be best effected by 
varying the distance between the lenses of the illuminator, or by 
stops or diaphragms external to the illuminating eye-glass, I have 
not yet tried, or whether there should be a variety of eye-glasses for 
the various powers. 
‘‘In my present illuminator the spot of light is about once and 
a half the diameter of the field of my inch power ; yet with half- 
inch or quarter-inch objectives it gives such views of vegetable tis- 
sues, of fossil woods or teeth, and such like, as I have never other- 
wise seen, whether as regards distinctness and manifest truthfulness 
of details, or neatness of definition of the exterior edges; and with 
one-eighth objective of 108° aperture it, by direct light, distinctly 
shows “both sets of lines on several of the more difficult test na- 
vicule, separating some of them into dots; but on this class of ob- 
jects I have as yet done so little that I am unwilling to go into any 
details. Corroborative of the value of parallel light for illumination, I 
may here refer to a recent experiment. An achromatic microscope 
was directed to the minute but intensely brilliant image of the sun, 
formed by a solar microscope twelve or fourteen feet distant ; here 
the rays could have had but a very few seconds of divergence, but 
the most minute details were shown with exquisite definition. 
“With respect to the other mode of dealing with the useless 
rays before referred to, causing them to enter so obliquely as to pass 
entirely across the axis, and thus prevent their reaching the eye 
at all, I considered that it would be of value if not only the amount 
of obliquity but also the azimuth of the oblique ray in reference to 
the object could be varied by the observer, my impression being 
that in this manner many characteristic features of structure might 
be developed which with direct light could be seen but with diffi- 
culty, if at all: such as diseppiments in cellular tissues, elevations 
or depressions on the surface, or suchlike. I believed these would 
be rendered visible by the shadows they would cast. To effect this 
I at the time (ten or twelve years since) designed an instrument 
which would wholly revolve round the illuminated object as a cen- 
