June 23, 1924.
Dear Heidelberger:
Your letter written from London about the middle of May deserved a more prompt reply.  I was very much pleased to hear from you, and to learn that you were enjoying London and that your wife and baby were well.
You seem to have been very industriously occupied in visiting galleries and laboratories, and all you tell me of the latter interests me very much.  I am particularly interested in what you say of the possibility of having x-ray cryatallograms made of your oxyhaemoglobin crystals.
Wright at St. Mary's stands a good deal alone, I fear - he and his little group of disciples.  He is an extra-ordinary man, brilliant; but it is I suppose questionable whether the methods he personally employs so well are sufficiently quantitative to be generally applied.  It may indeed very well be that in some modified form they will come into use in time.
In the meantime, the base of immunology is changing somewhat.  It is drawing more heavily on chemistry - organic and physical - as you well know, and new phenomena, such as bacteriphage, etc. bring about new points of view and directions of work.  It is perhaps as well that we are diverse in our interests, for in this way different things and methods come to be tested and progress is accelerated.
By this time you have moved further on, to the continent probably.  I can imagine how full of interest you, your wife, and sister-in-law find all your experiences.  I hope you all remain well.  My wife and I send you all very warm regards.
Yours sincerely,
Simon Flexner