posed, the whole refraction will remain unchanged. 3. At the vertex
of a given triangle to place a given refracting surface, so that the in-
cident and refracting rays may coincide with the two sides of the tri-
angle joined at the vertex. 4. In oblique refractions at spherical sur-
faces, the line joining the conjugate foci passes through the point
where a perpendicular from the centre falls on the line bisecting the
chords cut off from the incident and refracted rays. 5. To find the
place and magnitude of the image of a small object after refraction
at any number of spherical surfaces. 6. To determine the law by
which the refraction of a spherical surface must vary, so as to collect
parallel rays to a perfect focus. 7. To find the principal focus of a
sphere or lens, of which the internal parts are more dense than the
external. And lastly, to find the nearer focus of parallel rays falling
obliquely on a sphere of variable density. How these various propo-
sitions, both problems and theorems, apply to the structure and func-
tions of the eye, will be manifest to those anyways acquainted with
investigations of this nature.
As the focal distances of the eye, whether permanent or variable, must be one of the principal data upon which this inquiry is to pro- ceed, an instrument for readily determining these distances could not but be a very essential desideratum. Although due praise be here given to Dr. Portenfield's optometer, invented for that purpose, Dr. Young, thinking it capable of considerable improvements, describes another apparatus of a more simple construction, and much more convenient and accurate in its application. Its principle depends on tbe cir- cumstance, that when we look at any object through two small boles witbin the limits of the pupil, if the object be at the point of perfect vision, the image on the retina will be single; but in every other case the image, for reasons previously stated, will become double, and will appear as two lines crossing each other in the point of perfect vision. Thus we see that this point of intersection coincides with that of perfect vision, and by the help of a lens, and of a scale deduced from one of the corollaries of the fourth proposition, we are enabled to determine the focal distance of every eye. The mechanical part of this apparatus must be learnt from the figures which accompany the lecture.
On these principles, and with this instrument, the author proceeds next to investigate the dimensions and refractive powers of the human eye in its quiescent state, and the form and magnitude of the picture which is delineated on the retina. This he has performed chiefly on his own eye; and he has in general grounded his calculations on the supposition of an eye nearly similar to his own. The various ex- pedients he has used for obtaining accurate measurements, is perhaps not the least interesting part of the lecture. Nor will the series of general observations on the structure and functions of the eye, into which the author enters circumstantially, be found of less moment and curiosity. Among these may be noticed the obliquity of the uvea, and of the crystalline lens nearly parallel to the uvea, with respect to the visual ray, whereby a distortion of the focal point is produced
