by the unsatisfactory appearance of the image that 

 something is amiss, but will probably not know how 

 to correct the deficiency. 



Using a 1-5, 1-6 or 1-8 inch objective and the test 

 object Pleurosigma angulatum, select a diatom which 

 is flat and locate in 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 accepted as being correct. 

 If 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-correc- 

 tion and the remedy is found in the contraction of the 

 tube length. This should be done progressively in 

 spaces of about one-half inch, and after each change 

 carefully 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 slightly oblique illumination. 

 If too much is used the nice differences will be lost. 



'39 



