POND LIFE 247 



The colonies on plate cultures are photographed in a 

 similar manner, illuminated from the front or from behind 

 with reflected light, or by means of condensers placed 

 at suitable angles. Proper contrast for light - coloured 

 colonies is obtained by supporting the plate or Petri dish 

 some distance in front of a black background and illumi- 

 nating from the front. Small colonies are sometimes 

 displayed to the best advantage by dark ground illumina- 

 tion with objectives of f" to 2" focus. For this purpose 

 the plate is laid on the microscope stage and a wheel-stop 

 placed beneath the condenser (Plate 4.B). 



Photography of Larger Micro-organisms in the 

 Living State, Pond Life, etc. (Plate 38). In order 

 to take a photomicrograph of the minute animals and 

 plants found in ponds, etc., whilst alive and perhaps moving 

 with considerable rapidity, instantaneous exposures are 

 essential, and some means for focussing and watching the 

 organisms right up to the moment of exposure must be de- 

 vised, in order that they may be taken at the moment when 

 properly expanded and in a good position in the field. 

 A reflex camera fitted up in the way described on page 13 

 is well adapted to the purpose, but the Leitz demon- 

 stration eyepiece shown at Fig. 71 is in many respects 

 even better. The special eyepiece is placed in the micro- 

 scope tube like an ordinary eyepiece. It has a lateral 

 tube along which some of the image-forming rays are 

 directed by a reflecting prism through a second eyepiece, 

 giving an image which can be examined direct, or pro- 

 jected on to a suitable surface and there focussed right 

 up to the moment of exposure, and watched during 

 exposure if desired. There is no chance of vibration from 

 the movements of a mirror, and no resetting required as 

 in the reflex camera. In the figure the card on which 

 the image is focussed is fixed in a clamp ; with a hori- 

 zontal camera the card or paper screen may be placed 

 on the table. 



