CH. VIII} PHOTO-MICROGRAPHY 241 



ENLARGEMENTS ; LANTERN SLIDES ; PHOTOGRAPHING 

 BACTERIAL CULTURES 



g 389. Enlargements. As a low power objective has greater depth of focus 

 or penetration than a higher power (\ 34), it is desirable in many cases to make a 

 negative of an object with considerable depth at a low magnification, and then 

 to enlarge this picture to the desired size. As a rule negatives will not bear an 

 enlargement of more than five diameters. 



For this work the camera shown in Fig. 181 is excellent, and the special mi- 

 croscope stand shown in this figure and in Fig. 175 serves to enable one to get a 

 very exact focus. 



One must select an objective for the enlargement with a field of sufficient size 

 to cover the part of the negative to be enlarged. An objective of 60 to 100 mm. 

 focus will answer in most cases. 



For the illumination the camera can be elevated against the sky, or artificial 

 light may be used. It is not easy to light so large a surface evenly by artificial 

 light. 



(A) Enlargement on Bromide Paper. For this the negative is put in place 

 and by pulling out the bellows the proper amount, one gets the right magnifica- 

 tion. Focus now as for any other object, using the fine adjustment and focusing 

 glass. 



For great exactness one must put a clear glass in the plate holder and focus 

 on the surface away from the objective. Then place the bromide paper on this 

 clear glass and put another over it to hold it flat against the first plate of glass. 

 The sensitive surface will then be in the exact plane of the focus and the picture 

 will be sharp. 



For the development and subsequent treatment of the paper, follow the 

 directions of the makers. 



(B) Enlargement on a Glass Plate. One may proceed in enlarging as for 

 making lantern slides and make a positive on a glass plate. If it is then desired 

 to get a negative for printing, place this positive on the microscope stand and 

 make a negative from it as if it were an object. Or one may make a contact im- 

 pression as is frequently done in lantern slide making. By this method one must 

 make three separate pictures, (i)the original photo-micrographic negative ; (2) 

 the enlarged positive from this ; (3) a negative from the enlarged positive. With 

 this negative one may print as from the original negative. 



\ 390. Lantern Slides from Negatives. In preparing lantern slides from 

 photo-micrographic or ordinary negatives one may use the contact method, or the 

 camera. With the camera one can enlarge or reduce to suit the particular case. 

 The camera and special microscope stand shown in Fig. 181 are admirable for the 

 purpose. For lantern slide work a photographic objective is used and the cone 

 for enlargement removed. One may put the objective in the front of the camera 

 or in the middle segment, making use of the little side door. 



\ 391. Photographing Bacterial Cultures in Petri Dishes. For the successful 

 photographing of these cultures dark ground illumination is employed on the 

 principal stated in 92. That is the preparation is illuminated with rays so 

 oblique that none can enter the objective. These striking the culture are reflected 



