(1676) Also , these rods may increase in hardness when they become mixed with the salty fluid always present on the eye - ball .From this I concluded that I had discovered the cause of the stinging pain caused by an eel ' s blood in the eye ; that is to say that the rods not only hurt the sensitive parts of the eye , but that some of them also remain sticking in it .In my letter of November 27th 1676 I wrote that I had dissected lice , and taken from them eggs , which we call nits , and from these eggs again lice .