(ap’-er-chur) An opening. A., Angular, in the microscope, the angle formed between a luminous point placed in focus and the most divergent rays that are capable of passing through the entire system of an objective. A, antri, the aperture of the umbrella cavity in medusae. A., Numerical, the capacity of an objective for admitting rays from the object and transmitting them to the image. A common measure is obtained as proposed by Abbe, by taking the product of the half angle int the retractive index of the medium employed. It is usually expressed n. a. =- sin u., n the index of refraction and it the semi-angle of the aperture. A. of Objective, the capacity for receiving and transmitting rays.
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