. The microscope; an introduction to microscopic methods and to histology. Microscopes; 1899. CH. VIII.] /Y/0 TO-MICROGRAPHY. 195. Fig. 149. Zeiss' Apochromatic, Projection Objective of 70 mm. equivalent focus, for photo-micography. (Zeisss' Catalog. Nr. 30), Th is, and another of 33 mm. jbcus, are designed for making pictures of moderate magnification. Usually rather large objects are photographed with them. The object may be il- luminated in the ordinary way. They are used without an ocular, like a photographic objective. The one of 33 mm. is screwed into the tube of the microscope like an o


. The microscope; an introduction to microscopic methods and to histology. Microscopes; 1899. CH. VIII.] /Y/0 TO-MICROGRAPHY. 195. Fig. 149. Zeiss' Apochromatic, Projection Objective of 70 mm. equivalent focus, for photo-micography. (Zeisss' Catalog. Nr. 30), Th is, and another of 33 mm. jbcus, are designed for making pictures of moderate magnification. Usually rather large objects are photographed with them. The object may be il- luminated in the ordinary way. They are used without an ocular, like a photographic objective. The one of 33 mm. is screwed into the tube of the microscope like an ordinary objective, but the one of yo mm. here shown, is, bv means of a conical adapter, screwed into the ocular end of the tube. Tor illuminating the object, any suitable light may be used, but it is recommended that the light be concentrated by means of a bull's eye or some form of combination like the engraving glass, and that the condenser be so placed that it focuses the light upon the objective, not upon the object. The object is then illuminated zoitli a converging cone of light. For the clear screen, Mr. Walmsley and others have recommended that a pencil mark or cross be made in the center of the ground glass, and then that a large circular or square cover-glass be put on the ground glass with Canada balsam. To do this, warm the ground glass carefully, add a drop of rather thick balsam to the center on the ground side, then apply the cover and press it down firmly. After the balsam has cooled it may be cleaned off around the cover with xylene or alcohol. The balsam will fill up the inequalities in the glass and being of about the same refractive power will make this part of the glass clear as if it were unground (Fig. 150). For using the focusing glass first carefully adjust it so that the pencil cross in the center of the ground glass is in the best possible focus. The image when in the best focus must then be in the same plane as Fig. 150. Focusing screen with clear ce


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