CH. VIII.] 



PHO TO-MICROGRAPHY. 



195 



Fig. 149. Zeiss' Apochroniatic, Projection Objective of 70 

 mm. equivalent focus, for pkoto-micography. (Zeisss' Catalog. 

 Nc. jo), This, and another of 33 mm. focus, are designed for 

 making pictures of moderate magnification. Usually rather- 

 large objects are photographed with them. The object may be il- 

 luminated in tlie ordinary way. They are used without an ocular, 

 like a photographic objective The one of 35 mm. is screwed 

 into the lube of the microscope like an ordinary objective, but 

 the one of jo mm. here shown, is, by means of a conical 

 adapter, screwed into the ocular end of the tube. 



For illuminating the object, any suitable light may be used, 

 but it is recommended that the light be concentrated by means 

 of a butt'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 with a converging cone of light. 



For the clear screen, Mr. Walrasley 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 xjdene 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 center 

 for the final adjustment with a focusing glass 

 like that shown in Fig. /jj or 75/. 



1. The ground surface of the focusing screen; 

 it is translucent but not transparent. 



1. Central clear part of the screen made by 

 cementing a cover-glass to the ground surface 

 with Canada balsam. In the center is shown 

 the pencil mark to indicate the plane to which 

 the focusing glass should be adjusted. 





1 







2 

 X 











the ground side of the focusing screen. If the uncovered part of the 

 focusin^ screen is too opaque, rub some fine oil on it ; only a little 



