PHOTOGRAPHING BACTERIA. 105 



without the use of an eyepiece ; and, as a rule, it is best not to use 

 an ordinary eyepiece to secure increased amplification, as this is ob- 

 tained at the expense of definition. But an amplifier may be used in 

 the tube of the microscope, as first recommended by Woodward. In 

 this case the amplifier must be carefully adjusted with reference to 

 the distance of the screen, to secure the best possible definition. 



The projection eyepieces of Zeiss are constructed especially for 

 photography and possess a decided advantage. By the use of his 

 three-millimetre apochromatic oil-immersion objective and projec- 

 tion eyepiece No. 3 we may obtain an amplification of one thousand 

 diameters with excellent definition. 



Light. Sunlight is in many respects the most satisfactory for 

 photography, but has the disadvantage that it is not always available. 

 In some sections of the country weeks may pass without a single 

 clear day suitable for making photomicrographs. In addition to the 

 uncertainty arising from cloudy weather, we have to contend with 

 the fact that the sun is only available for use with a heliostat for a 

 limited time during each day, and that this time is greatly restricted 

 in Northern latitudes during the winter months. When sunlight is 

 to be employed the microscope and camera must be set up in a room 

 having a southern exposure on a line corresponding with the true 

 meridian of the place. The heliostat is placed outside the window in 

 such a position that when properly adjusted the light of the sun will 

 fall upon the condenser attached to the substage of the microscope. 

 The condenser must be carefully centred, so that the circle of light 

 falling upon the screen shall be uniform in intensity and outline. 



The calcium, magnesium, or electric light may be used as a sub- 

 stitute for sunlight, but they are all rather expensive, unless, in the 

 case of the electric light, a suitable current is available without the 

 expense of generating it for the special purpose in view. The writer 

 has obtained very good results with the calcium light, but has no ex- 

 perience in the use of the electric light. Woodward, as a result of 

 extended experiments, arrived at the conclusion that " the electric 

 light is by far the best of all artificial lights for the production of 

 photomicrographs. " He used a Grove battery of fifty elements to 

 generate the current, and a Duboscq lamp. The current from a 

 dynamo would no doubt be much cheaper and more conveniently 

 used, if an electric-lighting plant was in the vicinity. 



The apparatus shown in Fig. 73 was designed by Mr. Pringle for 

 the use of the calcium light. It will serve to illustrate the arrange- 

 ment of the microscope and camera in connection with any other 

 light as well. An oil lamp may be placed in the position of the oxy- 

 hydrogen burner ; or, if sunlight is to be employed, a heliostat will 

 be placed in the same position. 



