164 BACTERIOLOGY. 



It is best to begin with the use of a low power, and a trial 

 object, such as the blow-fly's tongue, spiiie of Echinus, or trachea 

 of silkworm. 



In order to explain tlie management of the apparatus (as 

 represented in Fig. 78) the steps in the ari'angement of the 

 apparatus and exposure of the plate will be described in detail 

 for the employment of a high power and the oxyhydrogen light. 

 The solutions being ready for use, it is proposed to take a photograph 

 of tubercle bacilli in sputum, with a J^- apochromatic oil-immersion 

 objective. The first point to claim attention is the ai'rangement 

 of the light.^ Having lighted the gas at the hydrogen jet, the 

 lime-cylinder should be revolved until heated equally all round. 

 The oxygen is then carefully turned on until only a small spot 

 of incandescence is produced. The central sliding-board is turned 

 out, a low power screwed on to the microscope, and the image of 

 the bright spot focussed and accurately centered. To protect the 

 sight, an eye-piece provided with a smoked glass shade is used. 

 The immersion objective is then substituted for the low power, 

 and the oxygen turned on until the right admixture of gas is 

 obtained to produce a brilliant illumination. It is well at this 

 stage to sit down to focus the selected object, and to spend some 

 little time in searching for the most characteristic part of the 

 specimen to be photographed. This being decided upon, the eye- 

 piece is carefully withdrawn, and the central sliding-board rotated 

 back into the median line. To make a light-tight connection 

 between the camera and the' microscope, the camera is pushed up 

 until a velvet-lined tube, which occupies the position of the lenses 

 of an ordinary camera, is enclosed within a short wide tube which 

 is adapted to the eye-pieca eud of the microscope. 



On opening the camera-shutter the image will be projected upon 

 the ground glass screen of the camera. It is necessary, however, 

 to obtain the exact focus, and to effect this the ground-glass 

 screen is turned away, and the dark-back with a piece of plain 

 glass is substituted. Here again time may be well spent in 

 getting the sharpest image, with the aid of a focussing glass of 

 proper focal length. 



The greatest delicacy in manipulation is necessary, as in working ' 

 with such high powers a turn too much of the fine adjustment will 

 cause the image to vanish. Having determined the best visual 

 focus, which will be found with the high-power objectives of most 

 makers to correspond with the chemical focus, the dark-back must 

 be cautiously removed, to. prevent any vibration, and the plain 



