DARK GROUND ILLUMINATION 5 



or mucous patches, may be obtained by the use of dark ground illumina- 

 tion. There are many different types of apparatus for this purpose. 



The bacteria or spirochaetes are intensely illuminated and show as 

 brilliant silvery objects in contrast to the dark background. 



When the morphological details of a brightly illuminated object in the dark 

 field can be distinctly observed it is proper to use the term dark ground illumina- 

 tion. When only particles, usually surrounded by bright and dark rings, and not 

 showing any structure, are observed in the dark field the proper designation is ultra- 

 microscopic. An apparatus using only the short waves of the ultra-violet spectrum 

 enables one to observe particles no larger than i/io of a micron. For this appa- 

 ratus it is necessary to employ photographic plates. In using the i/i 2-inch ob- 

 jective with dark ground illumination a funnel-like base is supplied on which we 

 screw the nickle plated front mount of the objective. Before using the dark- field 

 apparatus it must be centered with a low power. This is carried out by getting 

 concentric rings parallel with the circle of the microscopic field. Immersion con- 

 tact b.etween the front surface of the Abbe condenser and the under surface of the 

 slide carrying the preparation must be made before focussing the i/i2th objective. 

 As a source of illumination we may use a small arc-lamp or a Nernst lamp or an 

 incandescent gas lamp. In using an arc-lamp one must have a suitable rheostat 

 according to the electrical current employed. Information as to voltage and nature 

 of current must be given the one supplying the apparatus. 



In making preparations the slides and cover-slips should be scrupulously clean 

 and the material thinly spread out and free of bubbles. 



APPARATUS FOR STERILIZATION. 



For the purpose of sterilizing glassware, media, and old cultures 

 there are three methods ordinarily employed. The hot-air sterilizer, 

 in which a temperature of about 150 C. is maintained for one hour, is 

 ordinarily used for the sterilization of Petri dishes, test-tubes, pipettes, 

 etc. If the temperature is allowed to go too high, there is danger of 

 charring the cotton plugs and also of causing the development of an 

 empyreumatic oil which makes the plugs unsightly and causes them to 

 stick to the glass. Again we must be careful not to open the door until 

 the temperature has fallen to 60 C., otherwise there is danger of crack- 

 ing the glassware. Where gas is not obtainable, the hot-air sterilizer is 

 not a very satisfactory apparatus. 



The Arnold sterilizer is to be found everywhere and can be used on 

 blue-flame kerosene-oil stoves as readily as with gas burners. The 

 most convenient form, but more expensive, is the Boston Board of 

 Health pattern. The ordinary pattern, with a telescoping outer por- 

 tion, answers all purposes, however. In the Arnold, sterilization is 



