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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 the cir- 

 cumstance, that when we look at any object through two small holes 

 within 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 



