65. Harley S^{t <1>}
Wednesday Evening

Dear Sir

I am extremely obliged to you for your reference to the description of the Actinometer, ^<2> the more so that D^r Lindley ^<3> had previously inquired of several scientific persons without success –

I have thought of a method of measuring the refrangibility of definite rays, which I think cannot fail of possessing sufficient accuracy and facility – If you will make the homogeneous rays I will have the instrument constructed, so as to give it a fair trial –

Yours very truly
H. F. Talbot

C. Wheatstone Esq
20 Conduit S^t

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Notes:

1. Harley Street, London.

2. Actinometer, an instrument for measuring the intensity of light. The term was coined by Sir John Herschel in ‘Additional Observations on Leslie’s Photometer, &c By William Ritchie, A.M. Rector of the Academy at Tain. In a Letter to Dr Brewster’, Edinburgh Journal of Science,v. 3, 1825, pp. 104–107).

3. Prof John Lindley (1799–1865), botanist.