212 



PHO TO-MICROGRAPHY 



[ CH. VIII 



magnification usually required varies from natural size (X i) to five 

 times natural size ( X 5) up to X 20. As with the large camera the 

 position of the ground glass for each magnification and for each objec- 

 tive is determined once for all by using a scale in millimeters. The 

 various positions are accurately noted, then one can set the camera 

 almost instantly for the desired magnification. The supporting rod is 

 divided to half centimeters and therefore the exact position is easily 

 recorded (Fig. 183). 



Fig. 176. Zeiss Micro-Planar for photographing 

 with low magnification and for projection (see Ch. IX). 

 These are made from 20 to 100 mm. equivalent focus, 

 those of 20 and 35 mm. equivalent focus have the standard, 

 Royal Society Screw, the others have a larger screw in- 

 order that the image may not be restricted. ( Cut loaned 

 by Bausch & Lomb Optical. Co.) 



§ 345. Special Microscope Stand. — For the accurate focusing 

 necessary for embryos one should possess a special microscope 

 stand with the stage in two or three stories and attached to the arm in 

 place of the tube of the microscope. The stage proper is absent. 

 This arrangement of the stage permits the use of the coarse and fine 

 adjustment of the microscope to be used for focusing. The position of 

 the camera on a low table (45 to 50 cm. high; makes it possible for the 

 operator to stand on the floor while making all the adjustments of the 

 the embryo and for focusing ; and all the parts are within reach 

 (Fig. 175). 



§ 346. Arranging the Embryos.— As usually prepared the em- 

 bryos are white and therefore require a dark background. This may 

 be attained either by placing the embryos in a dark dish or on some 

 paper blackened with water-proof India ink, or by/ putting them in a 

 glass vessel like a Petri dish, and a piece of black velveteen on the 

 stage below\ The specimens will of course be in a liquid, usually alcohol. 



Fig. 177. Wide angle anastigmat 

 objective for photographing at low mag- 

 nification (Bausch & Lomb Optical Co.). 



If several embryos are to be 

 taken at once, the embryos are ar- 

 ranged in rows something as the 

 words on a line. Arrange them 

 in even vertical as well as horizontal rows so that when the print 

 is made it will be easy to cut them apart. When the embryos are ar- 



