20 Sir David Brewster on certain A ffections of the Retina. 
where 6 and ¢ are estimated in the same direction. To find the 
points A and B where the ‘ripple cuts the circle, » must be re- 
placed bya in these equations; on squaring and adding the results, 
it will be found that 
either $=0, or d=24, ; 
the former has reference to the point A, and the latter to the 
point B. Substituting the latter value of ¢ in (39), we have in 
virtue of (37), 
tan (26,—@) = pee ape 
eh? satan". —4) 
whence we conclude 0=26,; thatis to say, the radius OC to the 
cusp bisects the angle formed by the radu OA, OB to the two 
points where the ripple enters and leaves the circle. 
Lastly, when a=1 or X=u, the two first of equations (33) 
reduce themselves to the system (28), which, as we know, repre- 
sents the involute of the circle. This verification of the result 
contained in art. 13 may be most easily made by putting a=], 
/&=0, and hence R=0O, in the equations (25), which are equiva- 
lent to the system (33). 
II. On certain Affections of the Retina. 
By Sir Davip Brewster, K.H., D.C.L., F.RS* 
1 fa three articles published in the Philosophical Magazine for 
1834+, I have described several affections of the retina, 
which since that time I have frequently had occasion to study. 
Mrs. Mary Griffiths of New York had observed “a reticulated 
or network structure upon opening her eyes for the first time in 
the morning{. At one moment,” she says, “the meshes of 
the network were of a dull brickdust colour, and the spaces 
between were of a pale dingy yellow; and in the next moment 
the case was reversed—the meshes and intersections were of this 
pale dmgy yellow, and the spaces or interstices were of a dull 
brick-colour.” She describes the meshes “as generally the fifth 
of an inch in diameter,” but without telling us the distance of 
the surface upon which this measurement was made. 
Hlaving believed it possible to determine experimentally 
whether a beam of light was a continuous stream like a stream 
of water, or a stream in which the parts were at such a distance 
as to maintain a continued impression on the retina, I received 
a narrow beam of solar light upon a white ground, and made a 
strip of white card pass rapidly back and forwards across the beam, 
= tan (26,—28,), 
* Communicated by the Author. 
Tt Phil, Mag. 1834, pp. 115, 241, and 353. t Ibid. p. 43. 
