Microscopic Magnification 



The magnification of the binocular microscope ranges from about 6X to 

 about 90 X. In the instruments used by most oil companies the upper magnifica- 

 tion limit is about 36 X. With ordinary two-tube fluorescent lamps for illumina- 

 tion, the maximum effective power is about 18 X, though most work is done at 

 powers ranging from 6X to 12 X. Higher powers are used only at the sacrifice 

 of breadth of field and proper illumination. 



It is important to decide at the beginning of the project the magnification 

 and illumination that will be used and then to make an effort to maintain these 

 conditions. Consistent interpretation and graphic representation is impossible 

 if the conditions under which the work is done are varied from time to time. 



Under a 9X combination of lenses, particles of 0.05 millimeter diameter 

 are distinctly visible. This magnification is quite satisfactory for most sample 

 work. Occasionally it is necessary to use higher powers for the determination 

 of lithologic details, but unless there is a special reason, high powers should 

 be avoided. 



Four good reasons for using the lowest power that will permit observing 

 the essential details of the rocks are (1) wider field of vision, (2) minimum of 

 eyestrain, (3) relatively higher illumination of the sample, and (4) greater 

 depth of focus. 



Illumination 



When the log is plotted concurrently with the sample examination, it is 

 highly important to provide adequate lighting for the log strip. Microscopists 

 seem to be in agreement that log plotting causes more eyestrain than does the 

 microscopic work. Proper lighting of the log must not be neglected. 



The two-tube fluorescent lamp is a popular type for low-power binocular 

 work. Although these lamps can be obtained in a number of models, the most 

 convenient one is a flexible — or jointed — stand drafting lamp that can be 

 clamped on the back side of the microscope table, thus leaving the table top 

 around the log and microscope free of obstructions. Such a lamp with 18-inch 

 tubes provides adequate light for both the microscope and the log. A narrow 

 shade on the top protects the eyes from direct rays of light. Experience has 

 shown that subtle shades of color are brought out best when one of the fluores- 

 cent tubes is blue-white and the other is flesh colored — the so-called daylight 

 tube. Pale tints of color in the rocks tend to wash out in the blue-white light, 

 and they assume a yellow cast when only the daylight tube is used. 



There are numerous types of focused stage lights, some of which are 

 satisfactory for well-sample work. However, when these lights are used, it is 

 necessary to provide additional lighting for the log strip. 



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