THE COVER-GLASS PROBLEM 113 



Correcting Oil-immersion Objectives. — For the best 

 results, everything else being correct, a good oil-immersion 

 objective must be corrected for cover-glass thickness by 

 altering the tube length slightly (even on the binocular). 

 It is sometimes preferable to have covers slightly less than 

 0.17 millimeter. This allows more working distance, and 

 allows pressure to be more readily applied to the object 

 (in immersion oil), since covers 0.14 miUimeter thick bend 

 more easily. 



For a 60 apochromatic objective of 1.3 aperture, corrected 

 for a 0.17 cover-glass, the writer finds by the star test that, 

 with hard crown-glass covers, the tube length must be 

 increased about an average of 2 millimeters for every 

 0.03 millimeter of decrease of cover thickness under 0.17. 

 This correction adds the final sharpness to the image. 

 Uncovered slides require an increase of about 10 millimeters 

 to give crisp images, with this objective. 



A 90 apochromatic of 1.4 aperture required an average 

 increase of about 1 miUimeter in tube length for each 0.01 

 of cover decrease under 0.17 millimeter. (Of course covers 

 thicker than 0.17 would require a decrease in tube length, 

 but such covers need not be employed.) 



When oil-immersion objectives are used with objects 

 in water, a slight increase in tube length is also needed for 

 the depth (which should not exceed 0.01 miUimeter) of the 

 object under water. So the total increase of tube length 

 in this case is more than if the object was mounted in 

 immersion oil, with the same thickness of cover. 



The required increase in tube length can be made in 

 a binocular by pulling out the eyepieces (presumably sprung 

 in) and applying a flexible scale. For the usual binocular 

 attachment (Bitumi), the required increase must be nearly 

 doubled. Its amount can be found by using the star test. 



Summary. — If a 20 objective is not substituted for the 

 40 objective, it is best to purchase covers of, or slightly 

 below, the correct thickness (unless the thinner and thicker 

 covers can be utilized in other ways). Correction by the 

 drawtube, and still more the use of the correction collar, 



