268 PHOTOMICROGRAPHY 



Fig. 102) with a Schuckert & Co. electric lamp, enabling the work to 

 be done at any time. 



Between the electric lamp and the microscope the following articles 

 are placed in a direct line on a sliding bar : 



1 . A Collecting Lens, diameter 4^ inches, consisting of an anterior and 

 posterior lens. The concave surfaces of these lenses are turned towards 

 the light, and adjusted so that the distance between the luminous centre 

 and the edge of the lens mount is about 4f inches, rendering the rays 

 nearly parallel, without introducing excessive aberrations. The anterior 

 lens is placed close to the lamp, and the posterior lens, which again 

 collects the parallel rays, on a point situated about 16 inches from the 

 anterior lens (the collecting lenses can be rotated). 



2. Between the anterior and posterior lenses an absorbing cell is 

 placed for the absorption of the heat rays generated by the electric 

 lamp. The walls of the cell consist of plate-glass discs about 4| inches 

 in diameter, while the distance between the glass discs is about 2 \ inches ; 

 an inlet tap is fitted on the bottom and an outlet tap on the top of the 

 body of the cell. (This cell can also be used with sunlight.) The cell 

 is filled with water, previously boiled, from the lower tap, because if 

 filled from above the almost inevitable result is cracking of the glass 

 discs. 



3. An Iris diaphragm is placed between the microscope and the 

 posterior collecting lens, with a total aperture of 2| inches, fitted with 

 spring clips for holding objects, coloured screens, and ground glass for 

 indirect illumination. 



The iris diaphragm is particularly adapted for centering the entire 

 apparatus, for shutting off false light, and when closed the illuminating 

 rays are focused in the small opening of the closed diaphragm (about 

 J an inch) by means of the posterior collecting lens. 



4. The microscope used with the large Zeiss apparatus is their special 

 stand for photomicrography (see Fig. 103), which is placed on an 

 adjustable sole plate with levelling screws, and instead of the ordinary 

 Abbe condenser, a special centering achromatic condenser is used (see 

 Fig. 104), with an aperture of TO, and is focused with respect to the 

 plane of the object by means of the rack and pinion movement of the 

 illuminating apparatus. The aperture of the illuminating pencil is 

 regulated by an iris diaphragm placed between the lenses of the con- 

 denser. The best lenses for photomicrographic work, especially for 

 bacteria and other high-power photography, are the apochromatic lenses 

 of Zeiss. The photomicrographs in this work, reproduced by the 

 colotype process, were made with the apochromatic homogeneous 

 immersion lens 2 mm., numerical aperture 1'40, while those reproduced 

 by half-tone process were made with a 1-5 mm. lens with a numerical 

 aperture of 1*30, and illuminated by means of a special heliostat of my 



