PRACTICAL EXERCISES WITH THE MICROSCOPE 263 



objective might be corrected in a similar way for the covers, 

 0.4 millimeter thick, used in counting blood corpuscles. 



34. Correcting High Dry Objectives for Uncovered 

 Objects. — Using the same objective as in 33, screw a very 

 low achromatic objective, or fit a suitable converging 

 achromatic lens, into the drawtube. Focus on an uncov- 

 ered nigrosin smear. Try (altering the drawtube) to get a 

 lens which nearly or quite corrects the objective for an 

 uncovered object, as shown by the star test. 



35. Tube Length of the Monobjective Binocular.^Test 

 the binocular, or the binocular attachment, for correct 

 tube length by the star test, using objectives previously 

 proved correct in the monocular microscope. Show that, 

 in the original form of the binocular (Jentzsch), the optical 

 tube length increases half as fast as the interocular distance 

 increases, with a range of about 1 centimeter. (This might 

 perhaps be remedied, as already stated, by having a spiral 

 fitting to each eyepiece tube, so arranged that putting 

 apart the eyepieces causes them to sink for the appropriate 

 distance.) 



36. Cover-glass Correction with Binocular.— Test the 

 40 dry objective of 0.85 aperture, or the 60 oil immersion 

 of 1.3 aperture, with a cover-glass 0.15 or 0.16 thick, 

 instead of 0.17 millimeter. Pull out the eyepieces (fixing 

 them with a cardboard or paper collar, if necessary) suffi- 

 ciently to correct the error. This will entail an increase 

 of about 10 millimeters in tube length for the 40 dry, and 

 less than 1 milhmeter for the 60 oil objective, for each 

 0.01 millimeter decrease of cover thickness. 



37. Magnifying the Eyepiece Circles. — Take a 10- or 

 16-times triple magnifier, and center it in a tight ring (of 

 card or metal) over the eyepiece, so that it can be readily 

 slipped on and off. Use it to magnify the eyepiece con- 

 denser and objective aperture circles, when arranging the 

 %, % or ^^0 cone for any particular objective and object. 

 It may be used with advantage even for the low-power 

 objectives. (Beck, Coles.) 



