MIRROR, STAGE, NOSEl'IECE, AND DRAWTUBE 111 



According to Messrs. Watson (130), the nosepiece should 

 always be turned round in the same direction, to wear well. 

 In Zeiss's nosepieces, this direction appears to be counter- 

 clockwise, seen from above. If there is any shake in the 

 spring catch at the centering point, the nosepiece should 

 be cleaned or repaired; for a wrongly centered objective 

 is a cause of poor images. Slight shifting of the spring 

 catch in some nosepieces may throw all objectives out of 

 center. Centering of the objective is of prime importance 

 for good work. In some nosepieces, probably through 

 wear, or through screwing up too tightly, the objectives 

 may over-ride the catch slightly and require to be brought 

 back by reversing the rotation. Otherwise, definition is 

 injured. 



The sliding objective changers of Zeiss, as already indi- 

 cated, are a good substitute for the revolving nosepiece, 

 in the hands of the experienced worker. They require no 

 more time to change than do eyepieces. 



Occasionally, the grooves of the nosepiece, where the 

 spring catches, should be freed from the dust which accum- 

 ulates there. It is advisable that the objectives should not 

 be unscrewed from the nosepiece except to clean the back 

 lens; and the microscope, with attached objectives, is best 

 kept under a bell jar with a vessel of fused calcium chloride. 



The whole of the work of the microscope depends on the 

 accurate centering of objectives on a good nosepiece; and the 

 best possible one should be procured (Coles). The objec- 

 tives, when once centered, should of course be retained in 

 the positions marked for them. 



The Drawtube. — If the objective has a correction collar, 

 as all high dry and water objectives should have, the draw- 

 tube is not needed; except to set the mechanical tube 

 length at 160 millimeters (or 170 millimeters with some 

 makers), measured from the shoulder of the objective 

 screw to the top edge of the tube. Even 1 millimeter 

 wrong sometimes is perceptible with high-apertured objec- 

 tives of long focus, and 3 millimeters may decidedly mar 

 the image with some objectives. The tube length as 



