248 DR. HUBERT AIRY ON A DISTINCT FORM OF TRANSIENT HEMIOPSIA. 
It is hardly necessary to indicate, in the outset, the broad distinction between Trans- 
ient and Permanent Hemiopsia. The latter, as the more important, has naturally 
received the larger share of attention from the ophthalmists, and a reference to their 
works will show its characters to be markedly different from those of the transient forms. 
In Dr. Bader’s recent work on ‘ The Natural and Morbid Changes of the Human Eye,’ 
p. 447, ‘Ilemiopia’ is clearly described as presenting three varieties, of which the first 
is the only one that comes into comparison with the transient forms, and the chief cha- 
racteristic of that first variety is concisely stated in these words : “ The line of demarca- 
tion between the sensitive and blind part of the retina is vertical and sharply defined.” 
This fact, together with the permanency of the graver form, and its association with 
intra-cranial tumours, apoplexy with hemiplegia, &c., will be found to offer strong points 
of distinction between the permanent and the transient forms of Hemiopsia. 
Most writers on the subject refer, as to a fountain-head, to Dr. Wollaston’s well- 
known paper “ On Semi-decussation of the Optic Nerves ” (Phil. Trans. 1824, 1. p. 222), 
in which he gives a graphic account of two attacks of ‘ half-blindness,’ with an interval 
of twenty years between them. 
On comparing his account with later descriptions, I think it will be recognized that 
Wollaston, Arago, and Brewster are describing one form, while Sir John IIerschel, 
Sir Charles Wheatstone, the Astronomer Royal, and Professor Dufour, with myself, 
are describing another (which had also been noticed by Dr. Fothergill and Dr. Parry). 
The striking facts of gradual increase, motion, form, and colour which characterize the 
records of the later group, could not have escaped the notice of the three earlier observers, 
if present ; and we must conclude from their silence that these features were wanting or 
at least Avere inconspicuous. 
In Dr. Wollaston’s paper, the passage of chief importance for comparison with other 
accounts is the following: — “This blindness Avas not so complete as to amount to abso- 
lute blackness, hut Avas a shaded darkness without definite outline. The complaint was 
of short duration, and in about a quarter of an hour might be said to be wholly gone, 
having receded Avith a gradual motion from the centre of vision obliquely upwards 
towards the left.” The author lays stress on the equal affection of both eyes, and 
concludes that corresponding tracts of the tAvo retinse receive fibres from the seat of 
disease. 
Wollaston’s paper attracted much attention abroad as well as at home. A full 
translation of it appears in the ‘ Annales de Chimie et de Physique ’ (tom. xxvii. p. 102), 
and the editor, M. Arago, appends a note in Avhich he illustrates the subject from his 
own experience. There can be no doubt that Arago and Wollaston describe the same 
affection, but in both accounts Ave miss the remarkable features that characterize the 
particular form of hemiopsia on which I shall chiefly have to dwell. 
