141 



something is amiss, but will probably not know 

 how to correct the deficiency. 



Using- an objective J, or -J- inch and the test 

 object P. angulatum, select a diatom which is flat 

 and move to the center of the field. Focus care- 

 fully so that the margin of the object will be 

 sharply defined and observe the markings. If 

 they show in the same plane without any further 

 focusing-, the spherical correction may be taken as 

 being correct. If, however, the lines appear to lie 

 in a higher plane and it is necessary to focus 

 upward so that the margin of the diatom is out of 

 focus, it indicates spherical over-correction and 

 the remedy is found in the contraction of the tube 

 length. This should be done progressively in 

 spaces of about \ inch and after each change, care- 

 fully focus again until proper correction is 

 obtained. 



When the lines appear to lie below the plane of 

 the object, it indicates spherical under-correction 

 and can be corrected by increasing the tube length. 

 If there are two or more eyepieces, results can be 

 obtained quicker with the higher powers. 



If the markings cannot be seen, it may be due to 

 abnormally thick or thin covers, a not uncommon 

 occurrence, thus destroying the resolving power. 

 This may be judged by using slight oblique illu- 

 mination. If too much is used the nice differences 

 will be lost. 



