Trinity College,Oct. 3rd, 1853.

Dear Sir,

What I said about chlorophyll was merely a vague remembrance of one out of a number of facts in Chemistry which Professor Gregory mentioned to me last Winter during a visit to his laboratory. I am very sure that he told me of its isolation but less sure about its colour. He gave me no reference to any source of information and at that time I had no intention of asking for any.

I have written to him on the subject, but I have not yet received an answer.

With respect to my conjecture about Haidinger's brushes, your statement of it is quite accurate and expresses all that deserves the name even of conjecture†. In August 1850 I noted down some experiments and deductions from them, some of which are the same with those which I have since seen described in the fourth part of Moigno's Répertoire d'Optique, particularly the experiment with a plate of quartz.

I have published nothing on the subject. In fact I have had no opportunity of obtaining sufficient foundation for a theory, and a conjecture is best corroborated by the existence of identical or similar conjectures springing up independently.