Measurement of Radiation Intensities by Photographic Methods

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REFERRING to Dr. F. C. Toy's letter, under the above heading, in NATURE of January 16, p. 83, Dr. John S. Anderson and I came to the same conclusion in an investigation published in the Proc. Roy. Soc. Edin. fourteen years ago. In the words of this paper, “Only when two beams of light of the same wave-length fall on adjacent parts of the same photographic plate and produce equal blackening in the same time can we say that their intensity is equal.” This principle was made the basis of all the work we did at that time on ultra-violet spectrophotometry. But there are some fields, such as astrophysics, where it cannot be applied.

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