25th March, 1957
Dear Dr. Gray,
Thank you for your letter of 20th March.
We have exposed one of the pieces is of perspex which you sent me, and I am now returning it to you.  The X-ray exposure was overall a length of 15 mm in the centre of the piece, and lasted 20 hours.  Because no collimator was used, the intensity would be slightly greater than that which reaches of virus crystal, but the total exposure would be about the same as one of the longest exposures of a crystal.  I should be very interested to know what does level this corresponds to.  I gather from a recent discussion with Dr. Perutz that 5 times 10 [to the fifth] may be an under-estimate.
Yours sincerely,
Rosalind Franklin