"To repeat here these ... observations, just as if I had never written about them."
In the previous Letter 41, Leeuwenhoek had explained:
I all along intended to examine the disease called leprosy by the common people of this country, but I was prevented from this partly by the long spell of cold weather, partly by my observations of the crystalline bodies in the eyes, especially those of birds, on which I spent a lot of time before I could discern the course of the fibers; and after that I turned my attention to the brains of turkeys, which in its turn was an occasion for me to observe the brains of other animals.
This Letter 42 recorded that sequence of investigations.
Being lately occupied in the examination of the eyes of turkeys, my eye lighted once more on the brain; and although in my letter of 14 May 1677 I communicated to you Noble Gentlemen my observations on the subject, I now find myself obliged to repeat here these, my later observations, just as if I had never written about them, all the more because I am of the opinion, that I can now see these things more distinctly and can deal with them better than before.
Over half of this letter was devoted to a description of the structure of the brain of turkey, sheep, oxen and sparrows. The first three would have been readily available at the markets in Delft. He included three calculations of the size of the blood vessels.
After a short follow-up on the structure of the lens in the human eye, the second half of the letter responds to other researchers on gout, debunks the claims of four people to suffer from leprosy, and explains how Jews have unnecessarily avoided eating eels.
Leprosy: "Such filthy ulcers, that the sight of them disgusted me."
In Leeuwenhoek's time, leprosy referred to a variety of skin conditions. He himself suffered from such a condition, as he described in Letter 40 of December 28, 1683. In this Letter 42 the following July, he called it his "former illness" (voorgaande siekte).
Although in many places in our country the Leper or Lazar houses have been done away with and many who have a knowledge of these things say that there are no sufferers from leprosy, who cannot be cured, nevertheless in Haarlem persons have been appointed, to whom the so-called lepers go, to show themselves or to be examined, from whom some of them receive a letter bearing a seal and a clapper, so that they can beg their bread during four successive years, after which they must be reexamined.
On the right is a photo of one of these clappers with the city of Haarlem's coat of arms. Leeuwenhoek told of meeting four of these beggars; he compared their conditions to his own.
Meeting one of these persons, who had been allowed to beg, I asked him what was the nature of his disease. He showed me his head, which was quite covered with peeling skin and dry red scabs which scabs I judged to be due solely to the fact, that when his head itched badly he had scratched it, till it bled and at that place the blood had dried (as has often happened to me under similar circumstances). As to the rest of the patient's body (he told me) it was quite whole.
The second leper so-called was a man of approximately 36 years of age, whose head and face were quite healthy; his head was covered with very thick hair. But the rest of his body (he told me) was covered with large and small white and red spots and also red scabs, which he showed me on his arms and legs. The white spots were the scales, which had been peeled off and shed by the skin, the red spots were where the scales had either been scratched or rubbed off. The red scabs were caused solely by the severe scratching, by which the blood had flowed from the skin and had there dried up, and had left a red elevated scab, what the patient also admitted to me. In short this disease did not differ from that of my former illness, except that the peeled off scales in his case were more numerous than in mine.
When he met two other beggars allowed to carry leper clappers, his examination did not get as far.
A third person whom I met and spoke with and who was under the Haarlem inspection, was a boy of about 12 years. His head was covered with such filthy ulcers, that the sight of them disgusted me.
A fourth person, whom I met, was a woman of nearly 30 years; she had also been allowed to carry a leper clapper to beg her bread. She said that her disease was nothing else than the scaling of the skin of her head, which was very itchy and when she scratched it, bled and was very painful and this was followed later by red scabs. She offered to uncover her head, but I was satisfied with what she told me, as I was staying in a strange town with friends.