. Carnegie Institution of Washington publication. PHOTOMICROGRAPHY. 139 portion of a considerable image of light. My custom is to nearly close the iris diaphragm below the Abbe condenser and throw with the condensing lens a small circle of light into the center of this diaphragm ; the condensing lens is then slid along the track about 12 or 15 inches nearer; the iris diaphragm is then opened wide and the exposure made at once by squeezing the bulb of the shutter. I now always use apochromatic lenses and never maKe negatives without an eye-piece. I have used Zeiss projection oculars, but now us
. Carnegie Institution of Washington publication. PHOTOMICROGRAPHY. 139 portion of a considerable image of light. My custom is to nearly close the iris diaphragm below the Abbe condenser and throw with the condensing lens a small circle of light into the center of this diaphragm ; the condensing lens is then slid along the track about 12 or 15 inches nearer; the iris diaphragm is then opened wide and the exposure made at once by squeezing the bulb of the shutter. I now always use apochromatic lenses and never maKe negatives without an eye-piece. I have used Zeiss projection oculars, but now use in preference a Zeiss. Fig. 127* No. 4 compensating ocular, or Spencer No. 3, which is kept solely for this purpose (so as to be always clean). It is of the utmost importance that mirror, walls of light-filter, alum-cell, and surfaces of condenser, slide, objective, and ocular be abso- lutely free from dirt, grease, and dust particles, even the smallest, if a good negative *Fic. 127.—Bacterial leaf-spot of the larkspur (Delphinium). Same as fig. 126, but photo- graphed on Cramer's isochromatic slow plate. In this photograph the black spots on a green back- ground come out distinctly; in fig. 126 they do not. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Carnegie Institution of Washington. Washington, Carnegie Institution of Washington
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