CH. VIII} PHOTO-MICROGRAPHY 209 



alphabetical order as are the catalog cards of books in a library. 

 One can then find any negative with the same facility that the title 

 of a book can be found in a card catalog. 



PHOTOGRAPHING EMBRYOS 



For photographing embryos and many other small specimens 

 it is more convenient to use a smaller apparatus than the vertical 

 camera just described. It is necessary also to have a more delicate 

 method of focusing. 



§ 279. Camera for Embryos. — This is a vertical camera for 

 photographing with the microscope, and with a photographic objec- 

 tive in the end of the camera as for an ordinary camera. This is 

 readily accomplished by having a society screw adapter, and also 

 adapters for the micro-planars or other objectives which one desires 

 to use. The magnification usually required varies from natural size 

 (x 1) 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 objective 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. 172). 



Fig. 166. Diagram of the general 

 construction of the Micro-Tessar ob- 

 jectives of the Bausch & Lomb Optical 

 Co. 



§ 280. Special Microscope Stand. — For the accurate focus- 

 ing necessary for embryos one should possess a special microscope 

 stand with the stage in two 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 adjust- 

 ments of the embryo and for focusing; and all the parts are within 

 reach (Fig. 165). 



