174 THE USE OF THE MICROSCOPE 



is below the object, while with the focus above the object, 

 there is a mist; and vice versa for overcorrection. The rule 

 is: undercorrection shows mist beyond focus, seen on 

 focusing up; overcorrection shows mist within focus, seen 

 on focusing down. If the rings are not circular, there is 

 astigmatism in the objective. If the rings are not con- 

 centric, the lenses of the objective are not properly centered. 

 Other aberrations alter the rings, when the tube length is 

 correct. With everything correct, they should be circular, 

 concentric, correctly spaced, and of nearly the same 

 brilliancy above and below the focus. 



12. Testing the Revolving Nosepiece. — The nosepiece 

 should be tested to see if all the objectives center nearly 

 the same and center in the tube when the nosepiece is 

 removed. They will not if the snap has loosened. The 

 test can be made without immersing any objectives. 

 The rotating part of the nosepiece should be screwed up 

 tight enough to prevent any shake on pressing on an 

 objective; but not so tight as to prevent the snap working 

 properly. 



