DARK-FIELD ILLUMINATOR 97 



micrometer, the draw-tube must be left exactly where it is. If it 

 had been drawn out say to 171.5 millimeters, as shown by its scale, 

 it must be left there after the removal of the stage micrometer and 

 the substitution of the slide with the bacteria. A large number of 

 the most typical forms (not involution forms) must be measured and 

 the size given in terms of the minimum and maximum length, for 

 instance, from 2.5 to 3.75 micra. 



Camera Lucida. The camera lucida drawing apparatus and the 

 photomicrographic camera are important microscopic accessories in 

 the study of bacteria. The use of the former is very simple and 

 explains itself; that of the latter, however, cannot here be taken up 

 in detail, since it requires special studies and much delicate work in 

 order to obtain good photomicrographs. When they are prepared 

 with skill they are by far the best method of illustrating micro- 

 organisms. 



Camera lucida drawing apparatus. 



Dark-field Illuminator. A microscopic accessory which has recently 

 come very prominently into use is the dark-field illuminator, devised 

 for observing living bacteria and other microscopic objects. It 

 shows these as the only light objects in an otherwise perfectly dark 

 field of vision. When it is to be used the condenser lens of the sub- 

 stage illuminating apparatus of the microscope must be removed and 

 the dark-field illuminator placed either on top of the stage or slipped 

 into the position that the Abbe condenser lens previously occupied. 

 Instruments are designed to work in either one position or the other, 

 but when constructed for use on top of the stage they cannot be used 

 below it, and vice versa. The dark-field illuminator is so arranged 

 that it will refract the parallel rays of light which are reflected upward 

 by the mirror, so that they will be changed into very oblique rays. 

 These, when they strike the cover-glass of the preparation, suffer what 

 is known as total reflection; that is, they are refracted back in the 

 direction from which they came; in consequence of this the field of 

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