252 DR. HUBERT AIRY ON A DISTINCT FORM OF TRANSIENT IIEMIOPSIA. 
thinking of nothing, when I was startled by a singular shadowy appearance at the out- 
side corner of the field of vision of the left eye. It gradually advanced into the field of 
view, and then appeared to be a pattern in straight-lined-angular forms, very much in 
general aspect like the drawing of a fortification, with salient and re-entering angles, 
bastions, and ravelins, with some suspicion of faint lines of colour between the dark lines. 
The impression was very strong, equally so with the eyes open or closed, and it appeared 
to advance slowly from out of the corner till it spread all over the visual area, and 
passed across to the right side, where it disappeared. I cannot say how long it lasted, 
but it must have been a minute or two. I was a little alarmed, looking on it as the 
precursor of some disorder of the eyes, but no ill consequence followed. Several years 
afterwards the same thing occurred again, and I recognized, not indeed the same precise 
form, but the same general character — the fortification outline, the dark and bright lines, 
and the steady progressive advance from left to right. I have mentioned this to several 
persons, but have only met with one to whom it has occurred. This was a lady of my 
acquaintance, who assured me that she had often experienced a similar affection, and 
that it was always followed by a violent headache, which was not the case with me. In 
this case the regularity of the pattern was not great, but the lines were quite straight 
and the angles sharp and well defined. Had it remained stationary, it might be 
assumed that the retina had a structure corresponding to the figure, and that some undue 
pressure might render that structure visible. But such an hypothesis is precluded by 
the gradual transit of the lines over every part of the visual area.” 
The following extract from a letter from Sir John Herschel to myself (May 4, 1868) 
will show that in his later experience the affection has begun near the centre of vision. 
“ It is very strange, and I am sure more than a coincidence, that two or three hours 
after I had read your letter, and while in the act of reading a printed book, I caught 
the impression of the commencement of an attack in the obliteration of one or two letters 
a little to the left and below the point of vision. Soon after, further out to the left in a 
wavy course, the printed letters ran into large angular black zigzags, and then I knew 
what was coming, and shutting my eyes I watched the development of the luminous 
bastions, &c. It was, however, by no means so well developed or striking an instance as 
I have had, but it is its recurrence evidently as a consequence of the mind dwelling 
on its description that I look on as worth notice.” 
Sir John Herschel has very kindly communicated to me his latest experience in a 
letter dated Nov. 17, 1809. 
“ Since I wrote to you I have been very frequently visited with the phenomenon — in 
a greater or less degree, — never, however, with the extreme vividness of colour and dis- 
tinctness of form as heretofore ; and it has assumed some new features, viz. patches of a 
kind of coloured chequer work in some of the corners of the fortification forms. 
“ It always now begins with a small glimmer near the middle of the field of view, and 
