THE MICEOSCOPE STAND 93 



factory with rapidly- moving macliinery, the microscope table 

 may be placed on a stone which rests on an inflated motor-car 

 tyre. Under ordinary circumstances such precautions are not 

 necessary, and any firm table is satisfactory. 



The stage should be firm and its upper surface should not be stage. 

 less than 4| inches above the table. An ebonite covering makes 

 a better surface than metal for giving a smooth motion to the 

 slide. It is less likely to be damaged by reagents, but a brass 

 stage with a surface ground flat is very satisfactory. There 

 should be a horizontal distance of not less than 3 inches 

 between the optic axis or centre of the stage aperture and the limb 

 to enable Petri dishes and large culture plates to be examined. 



The body tube must be of a variable length. The early Body tube. 

 microscopes were generally made with a 9- or 10-inch tube, but 

 have been entirely superseded by the more compact type which 

 has a tube length of 140 mm., which, by means of a drawtube, 

 can be increased to 200 mm. The shorter tube length has an 

 advantage in addition to the reduction in the size of the microscope 

 of which it admits. In the previous chapter it has been explained 

 how the variation in the thickness of the cover glass can be 

 largely compensated by a variation in the length of the drawtube. 

 A body tube of great length must be moved to a great extent to 

 produce much alteration, while a short body is far more sensitive 

 in this respect, and a much greater range of correction can be 

 obtained. It is also possible by an extra tube to further lengthen 

 a short body, while it is not feasible to shorten a long tube. 



The drawtube should always be graduated in millimetres, Drawtube. 

 which give the length at every position, and it should work with 

 great smoothness, so that when the microscope is in use the 

 length of the tube may be altered without exerting any force 

 which is likely to upset the adjustment of the instrument. The 

 sliding fitting should always be in cloth or other fabric which will 

 ensure a smooth motion. A metal-to-metal slide is not so satis- 

 factory ; such a slide may be perfect when it leaves the makers' 

 hands, but the slightest film of tarnish or oxidisation ruins its 

 working and gives a jerky, uneven motion. Due to the elasticity 

 of a thin cloth or a fabric slide, it cannot be quite as stiff and rigid 

 as a metal slide, but this is a matter of no practical consequence, 

 as a slight movement of the eyepiece out of the optic axis has 

 no effect on the quality of the image. The drawtube must be 

 provided with a diaphragm to prevent reflections at the inner 

 sides of the tube, and the upper portion of the drawtube should 

 be slightly smaller in diameter than the lower part, so that 

 pushing the eyepieces in does not tend to polish the tube below 

 the position where the shortest eyepiece fits. The lower end of 

 the drawtube should have a screw fitting for the use of a low-power 

 object glass. 



The coarse and fine focussing adjustments must be well made Adjustments. 



