same date. Metaphysics remained also, as it had ever been, a
favourite subject of Hamilton's readings and meditations and of
correspondence with his friends. He wrote a very long letter to Dr.
Ingleby on the subject of his "Introduction to Metaphysics." In it
Hamilton alludes, as he has done also in other places, to a
peculiarity of his own vision. It was habitual to him, by some
defect in the correlation of his eyes, to see always a distinct image
with each; in fact, he speaks of the remarkable effect which the use
of a good stereoscope had on his sensations of vision. It was then,
for the first time, that he realised how the two images which he had
always seen hitherto would, under normal circumstances, be blended
into one. He cites this fact as bearing on the phenomena of
binocular vision, and he draws from it the inference that the
necessity of binocular vision for the correct appreciation of
distance is unfounded. "I am quite sure," he says, "that I SEE
DISTANCE with EACH EYE SEPARATELY."

The commencement of 1865, the last year of his life saw Hamilton as
diligent as ever, and corresponding with Salmon and Cayley. On April
26th he writes to a friend to say, that his health has not been good
for years past, and that so much work has injured his constitution;
and he adds, that it is not conducive to good spirits to find that he
is accumulating another heavy bill with the printer for the
publication of the "Elements." This was, indeed, up to the day of
his death, a cause for serious anxiety. It may, however, be
mentioned that the whole cost, which amounted to nearly 500 pounds,
was, like that of the previous volume, ultimately borne by the
College. Contrary to anticipation, the enterprise, even in a
pecuniary sense, cannot have been a very unprofitable one. The whole
edition has long been out of print, and as much as 5 pounds has since