22 HOW TO SEE WITH THE MICROSCOPE. 



this test all the way of the "run" of the rack, and if 

 there be " lost motion" detected discard the stand. 



THIRD. Examine carefully the bearings on which 

 the body slides ; these should be broad, and the fitting 

 of the body to the limb accurate; test by placing a 

 small object on the stage a circular diatom will be the 

 thing; now examine with a half-inch glass; rack the 

 tube up and down a little, and see if the object keeps 

 its centre ; seize the body-tube near the eye-piece, and 

 twist it a bit from right to left, and vice versa, noticing 

 whether the object "travels" or not; ^epeat this ex- 

 periment by punching holes in a card-box, say three- 

 quarters of an inch thick, thus forming a supplemental 

 stage; focus again and try the twist once more. The 

 instrument that will stand this test is all right as to its 

 bearings. 



FOURTH. Place the diatom on the stage, under the 

 half -inch, as before, and if the instrument has fine ad- 

 justment by nose-piece, test by moving the fine wheel 

 quickly either way, and see if the object " travels." If 

 satisfactory with the half-inch, try an eighth, or tenth ; 

 test also by taking hold of the objective as one would 

 in the adjustment for cover, by giving it a little twist 

 right and left, and see if the object changes its position. 

 Finally, examine the run of the fine adjustment; see if 

 it is quick and sensitive to the touch, without "jump;" 

 if satisfactory so far, so good. 



FIFTH. Should the stand have concentric stage, test 

 the accuracy of its fittings with diatom and half-inch 

 glass. If the stand has suffered transportation, or has 



