74 THE A B C OF PHOTO MICROGRAPHY 



arrangements throughout are identical, the 

 increase in magnification being made solely by 

 camera extension to about five feet, decreasing 

 the illumination, but not to so great an extent 

 as might be supposed. Observe that in this 

 instance, as with all others in which an eye- 

 piece is used, the camera is detached from 

 microscope and slid back in order that center- 

 ing and arrangement of object and illumination 

 may be done directly on the microscope itself 

 in the same way as ordinary examinations are 

 made with that instrument. The camera is 

 then attached and lighting, focusing, etc., 

 completed upon the screen. For the present 

 exposure I will use a rapid landscape plate 

 not orthochromatic and the cobalt blue 

 screen. An exposure of ninety seconds, with 

 development by hydroquinon, gives us the 

 negative from which Fig. 12 was produced 

 at +3,000. 



Dark Field Illumination. There are many 

 transparent subjects which may be brilliantly 

 illuminated by pencils of light passing through 

 them at an obliquity too great to enter the 

 objective directly, so that they appear some- 

 what as opaque objects upon a black ground. 

 This is termed "dark field illumination," and 

 will frequently exhibit details not visible by 



