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Correspondence

Physics Department, The Queen Elizabeth Hospital, Edgbaston, Birmingham 15

This excerpt was created in the absence of an abstract.

In a recent paper Garrett and Jones (1962) reviewed three methods of deriving an isodose chart for oblique incidence from the appropriate normal incidence chart. These are:

(i) the effective absorption coefficient method (Greene and Tranter, 1956; Murison and Hughes, 1957);

(ii) the inverse square law method (Murison and Hughes, 1957; Day and Farmer, 1958).

(iii) The "half-way" method (Boland, 1958; Stewart, 1962).

It is sometimes implied that the third method is empirical, with no obvious physical basis. It can be shown, however, to rest on the same assumptions as do the other two.