242 PHOTO-MICROGRAPHY \_CH. VIII 



seconds, even with daylight. For metallic surfaces and magnifications of ioo, 

 150, 250 to 500, with the electric arc light as illuminant the time required for 

 favorable objects is 1, 2, 4 and 7 seconds; with the Wellsbach lamp the time is 

 5, 10, 30 and 60 minutes (Sauveur). 



ENLARGEMENTS ; LANTERN SLIDES ; PHOTOGRAPHING 

 BACTERIAL CULTURES 



\ 325. Enlargements. As a low power objective has greater depth of 

 focus or penetration than a higher power ($ 40), it is desirable in many cases 

 to make a negative of an object with considerable depth at a low magnifica- 

 tion, 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. 169 is excellent, and the special 

 microscope stand shown in this figure and in Fig. 165 enables one to get 

 an 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 artifi- 

 cial 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 

 magnification. 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 impression as is frequently done in lantern slide making. By this 

 method one must make three separate pictures, (J) the original photo-micro- 

 graphic 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. 



(i 326. 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. 169 are admir- 



