Here, considerable skill is also needed because many patients become quite tense when being tested in this way and might not initially give accurate answers. Lenses both stronger and weaker than the expected requirement are placed in the trial frames and the patient is asked to read the letters of the Snellen chart and to say whether they are more or less clear. A number of supplementary tests are available, which enable one to check the patients' answers. It can be seen that the word "refraction" refers to the total test for glasses, although the same word refers to the bending of the rays of light as they pass from one medium to another. Accurate refraction takes 10 min or 15 min to perform, or longer in difficult cases and it is an essential preliminary to an examination of the eye itself.
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