CARE OF Tiii<: Micnoscoi'E 181 



and lastly with filter paper moistened with distilled water, or 

 dry lens paper. The lens paper, or filter paper, is used 

 in little rolls, and the wiping is done with the fresh end of a 

 roll, as already noted. Objectives, when not in use, may 

 be stored in a dessicator over fused calcium chloride, and 

 may improve there instead of deteriorating. They may 

 be taken out after dessicating for some weeks, and screwed 

 tightly into their boxes, for storage. The brass work of 

 old objectives can be cleaned externally with a typewriter 

 eraser (and patience). They can then be coated with clear 

 cellulose lacquer. 



Care of the Revolving Nosepiece. — According to Messrs. 

 Watson (130), the nosepiece wears better if always rotated 

 in the same direction. In the writer's experience, this is 

 clockwise, as seen from below, with the revolving nosepieces 

 of Zeiss. The central screw sometimes needs a slight 

 tightening, for there should not be a perceptible shake. 

 The revolving nosepiece is one of the most important 

 mechanical parts of the microscope, and the best possible 

 one should be obtained. The screws of the spring catch 

 should not be loosened unnecessarily, as in some forms 

 the centering is done by shifting the position of the catch 

 a few hundredths of a millimeter. In some nosepieces 

 the final centering is done by taking advantage of a slight 

 lateral play when screwing the nosepiece to the microscope 

 tube. 



Care of the Drawtube. — There should be a diaphragm 

 in it, as Nelson pointed out. It should be ascertained by 

 measurement whether the graduations on the drawtube 

 include the nosepiece or not; and, if not, the proper allow- 

 ance should be made in setting the tube length at the correct 

 measure, usually 160 millimeters. The drawtube should 

 be polished occasionally, so that it does not slide stiffly. 

 A good drawtube should extend further below the 160 

 (or 170) mark than it usually does. 



Care of the Rackwork and Micrometer Screw. — The 

 former requires to be kept free from dust. Hence, the 

 instrument should be under cover when not in use, which 



