THE MICROSCOPE 149 



tube be not too long, the total tube-length may be reduced 

 to the proper amount by sliding in the draw-tube ; 

 (2) objectives are corrected for a definite thickness of 

 cover-glass, and if another thickness be used this should 

 be corrected for. In some lenses this is done by the pro- 

 vision of a " correction collar," but usually there is no 

 such adjustment, and the correction is effected by altering 

 the tube length for a thicker cover by shortening, and 

 for a thinner one by lengthening (see p. 167). For low- 

 power lenses these corrections are not so necessary, but 

 for high-power lenses and for all critical observations, 

 whether with low or with high powers, they are essential. 

 Alteration of tube-length also alters the diameter of the 

 field the longer the tube the smaller the diameter and a 

 draw-tube permits this alteration to be made, which is 

 very useful in blood work for certain counts. A distinc- 

 tion is made between mechanical tube-length and optical 

 tube-length. The former is the distance between the 

 shoulder of the objective when in situ on the microscope 

 and the upper end of the tube and is 160-170 mm. for 

 the short or Continental tube, the latter is the distance 

 between the upper focal plane of the objective and the 

 image plane, where the image of the object would be 

 found in the draw-tube were the eye-piece removed after 

 focussing. 



A sub-stage condenser is essential for all work in which 

 high powers are employed, and also enhances the value of 

 low powers. It consists of a sub-stage fitting carrying a 

 system of lenses below the stage, by means of which the 

 light is concentrated on the object. It should have a 

 rack-and-pinion, or a screw, adjustment for focussing, 

 and be provided with some form of diaphragm for modi- 

 fying the angle of illumination, preferably an " iris." 

 The condenser itself should be easily detachable so that 

 other sub-stage fittings may be inserted. The condenser 



