80 PRINCIPLES OF BACTERIOLOGY 



diaphragm, the aperture of which can be regulated, as circumstances 

 require, to permit of either a very small or a very large amount of light 

 passing to the object. 



Focusing. Focus the body tube down by means of the coarse adjust- 

 ment until the objective nearly touches the cover-glass, being careful 

 not to touch it. Then with the eye at the eye-piece focus up carefully 

 with the coarse adjustment until the specimen comes plainly into view. 

 Be careful not to pass by this focal point without noticing it. This is 

 likely to occur if the light be too intense and the specimen thin and 

 transparent. If the sliding tube coarse adjustment is used, focus care- 

 fully by giving the tube a spiral movement. 



When the object is brought fairly well into focus by means of the 

 coarse adjustment, use the fine adjustment to focus on the particular 

 spot desired, for if this spot is in the centre of the field of the 

 low power it will be somewhere in the field of the higher power. It is 

 too much to ask of the maker that the lenses be made absolutely par- 

 focal and centred. The delicacy of the centring can be appreciated 

 when the magnification and the extremely small portion examined are 

 considered. When the objectives are not thus fitted to the nose-piece, 

 refocusing and again hunting up the object are necessary. In so doing 

 we repeat the caution to always focus up before turning the nose- 

 piece. When no revolving nose-piece is used the change of objectives 

 means the unscrewing of one and the screwing of the other into its 

 place, and refocusing. 



The beginner should always use the low-power objectives and oculars 

 first. The low-power objectives have longer working distances and 

 are not so apt to be injured. They always show a larger portion of the 

 specimen and thus give one a better idea of the general contour. After 

 obtaining this general idea the higher powers can be used to bring out 

 greater detail in any particular part. 



Tube Length and Cover-glass. All objectives are corrected to a cer- 

 tain tube length (160 mm. by most makers Leitz, 170 mm.) and all 

 objectives in fixed mounts of over 0.70 N. A. are corrected to a definite 

 thickness of cover-glass as well (Zeiss, 0.15 mm., 0.20 mm.; Leitz, 0.17 

 mm.; Bausch & Lomb and Spencer, 0.18 mm.). These objectives 

 give their best results only when used with the cover-glass and tube 

 length for which they are corrected. As indicated in Fig. 53 the 

 tube length extends from the eye lens of the eye-piece to the end of 

 the tube into which the objective or nose-piece is screwed. If a nose- 

 piece is used the draw tube must be correspondingly shortened. If 

 the cover-glass is thinner than that for which the objective is cor- 

 rected, the tube must be lengthened to obtain best results; if thicker, 

 shortened. 



The more expensive objectives are provided with adjustable mounts 

 by which the distances between the lens systems may be changed to 

 compensate for difference of thickness of cover. They are success- 

 fully used only in the hands of an expert. One of them' out of adjust- 

 ment is worse than an ordinary objective. 



