PHOTOGRAPHY. 49 



picture " is lost. As the structural picture is lost, however, the stained 

 elements, such as the micro-organisms and the nuclei, are brought 

 out prominently and can be carefully examined. All preparations 

 should be examined with both the small and the large apertures be- 

 neath the condenser. With the small aperture, the structural elements 

 are observed and the positions of the organisms ; as the apertures are 

 increased in size, the organisms become more and more prominent, 

 whilst the structural elements gradually disappear from view. 1 Even 

 when no micro-organisms can be found in fluids and tissues examined 

 under ordinary conditions, they may still be present and may be 

 brought into view by the use of the above optical combination. 



Messrs. Powell & Lealand make an equally good combination, some 

 authorities even preferring their y^th in oil immersion to Zeiss' glass. 2 

 They also make a ^rth in oil immersion which is an excellent glass, 

 but such a power is of comparatively little use except for very special 

 work. 



Photography. 



35. Photography is now in many cases called to the aid of the ob- 

 server, and to help to record with accuracy what can be seen under the 

 microscope. Koch, by his admirable series of photo-micrographs, 

 has proved that most valuable results may be obtained by this method, 

 and that features which may escape observation under the micro- 

 scope come out prominently in a photograph, especially in enlarge- 

 ments. He uses wet or collodion plates, hence his apparatus, 

 consisting of Heliostat, condensers and the rest, is somewhat compli- 

 cated. For the ordinary worker the dry plate process gives sufficiently 

 good results, and with much less trouble and far simpler apparatus. 

 Koch works without an eye-piece, but this appears to be a matter of 

 detail, as many workers prefer to use it. We have found that with 

 an oxy-hydrogen light, or even with an incandescent electric lamp, it 

 is perfectly easy to use the eye-piece and a T Vth inch objective (Zeiss' 

 homogeneous immersion), with which arrangement it is found 

 practicable to have a comparatively short camera, and the necessity 



1 "Traumatic Infective Diseases," Koch, 1880. (German, 1878.) 

 '-' " Practical Histology and Pathology," Heneage Gibbes, 2nd ed., 1884. 



D 



