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Frederick H. Evans
Evan's Focusing Screen for Photomicroscopy
"Evan's Focusing Screen for Photomicroscopy" p.320 in the section "Summary of Current Researches Relating to Zoology and Botany, Microscopy, etc.", Journal of the Royal Microscopical Society, April 1887.
Evans's Focusing Screen for Photomicrography.* Mr. F. H. Evans refers to the difficulty which exists in focusing, by means of an ordinary focusing lens, the microscopic image projected on a screen of patent plate glass. This is due to the power of accommodation of the eye, in consequence of which the focal plane of the image is frequently assumed to be on the outer instead of the inner surface of the screen. He suggests that this difficulty may be readily overcome by ruling on the inner surface of the glass screen (i. e. the surface towards the Microscope) a series of fine lines similar to those shown in fig. 83; the eye has then before it a definite object in the focal plane upon which the focusing lens is adjusted, so that the almost involuntary movement of accommodation is practically arrested thereon, and tbe focusing of the microscopic image on that plane is thus greatly facilitated.
ò Journ. and Trans. Phot. Soc, xi. (1886) pp. 25-8 (1 fig.).