SOMETHING ABOUT AMERICAN STANDS, ETC. 23 



in any way been submitted to fatigue, the probability 

 will be that the stage has become somewhat decentred, 

 and any effort of your own to re-adjust will be likely to 

 make things worse. The very best stages will not 

 remain centred long if roughly used. Still, with your 

 half-inch and the diatom you can manage to get some 

 idea, which will be better than none at all. Centre 

 your diatom, and focus; now seize opposite edges of 

 the stage with both hands, hold it firmly, giving it 

 "jerks" either way, right and left, but not sufficiently 

 to absolutely move the stage on its centre. If you suc- 

 ceed in moving your object, with a corresponding move- 

 ment of the stage, then the latter is not accurately 

 fitted, and cannot be accurately centred. This "slip" 

 can be detected with the hands alone. 



SIXTH. It the stand be furnished with mirror fitted 

 to radial arm, see that all these fittings are strong and 

 likely to last. This portion of the stand meets, with 

 more fatigue than any other. See that the friction sur- 

 faces are not both the same metal ; this will apply, too, 

 to any part of the stand. Notice particularly whether 

 the universal motions of the mirror are properly con- 

 structed, and likely to last for years ; this is a most im- 

 portant point. 



FINALLY. It will be well to see that the joint for 

 inclining the instrument at various angles is strong and 

 well made, and that it have compensation for wear. 

 Notice the general ' get up" -the general finish of its 

 various parts. And now, having received a lesson on 

 the use of diatoms, you may put the little fellow away 

 until again wanted. 



