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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- 

 viculae, 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 such like. 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- 



