as the diameter of the field. Others would measure one of the longer sides of the plate, and the angle of view is usually esti-mated on this basis. A good deal of discussion has arisen as to the meaning of the term, " angle of view," but notwithstanding what has been said to the contrary, the simplest way out of the difficulty is to limit the term to the definition given above, and to explain the angle included by the lens on a given-sized plate as the picture angle. If this is adhered to we avoid several diffi-culties. Mr. W. Rice suggests the following simple formula : -
Let P = length of plate.
F =s focus of lens. Then
Or, expressed in words: - Multiply the base line of plate by 63 and divide the product by the length of focus, added to | of the diagonal of the plate.
Example: - What is the angle given by a 10-in. lens on a 12-in. plate ?
This formula will only apply when the angles included are less than 900. When the angle is greater than 900 the following should be used: -
Or, expressed in words : - Subtract from 180 the quotient obtained by dividing 126 times the focus of the lens bythe base line of the plate, added to twice the focus divided by 5.
Example: - What is the angle included by a 4-in. lens on a 12-in. plate?