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Abstract: The factors influencing the observational errors during precision instrument inspection were studied in a factory inspection center by the following methods: 1) Counting the frequency of the worker's eye-blinks; 2) Recording the worker's reaction time between the instant as he looked into the eye-piece of the optical micrometer and the recording of the results; 3) Administrating an aligning optimeter test, as designed by W. oMoede, before and after work.The results indicated that there were two kinds of errors in the inspection task, namely, aligning errors and negligent errors. The causes of aligning errors were: 1) Visual fatigue. The longer the worker engaged in such kind of strenous visual work, the higher the frequency of his eye-blinks; 2) Self-suggestion (preoccupation). As the worker acquired incorrectly the knowledge of results, the influence of preoccupation was the more obvious; 3) Emotional factor. The more the worker doubted of the inspection results, the more frequent the inspection errors; 4) Constant errors; and 5) The structure of the stimuli, such as length, direction and form of the instrumental scales, the contrast between the scales and background, etc. The causes of negligent errors were: 1) Fatigue of the second signaling system; and 2) Extra-stimuli, causing distraction.The author proposed a reasonable arrangement of the alternation of work and rest as well as the correct method of acquiring knowledge of results. The effect of using blinking as a measure of visual fatigue and the causes of constant errors were also discussed.

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