A NEW OPHTHALMOSCOPE. 



(3.) Parallel rays of light, incident upon a convex lens, are so re- 

 fracted that they leave the second surface of the lens convergingly 

 and meet in a focus at the principal focus of the lens. 



(4.) Conversely if rays of light diverge from a focus at the principal 

 focus of the lens, the corresponding refracted rays will be parallel. 



(5.) When diverging rays of light are incident upon a double con- 

 vex lens, and radiate from a point beyond its principal focuSj the cor- 

 responding refracted rays are brought to a focus on the opposite side 

 of the lens and at a point further from the lens than the principal 

 focus. 



(6.) Converging rays under the same circumstances form a focus at 

 a point between the lens and its principal focus. 



(7.) As I shall have occasion to use the word Camera in this paper, 

 I will here state that I refer to the instrument used in Photographing, 

 wtich consists of a darkened box, to one end of which is adapted a 

 tube containing one or more convex lenses of such strength that the 

 principal focus is within the box. Objects in front of the lens will 

 form an inverted image in the box which is usually received upon a 

 screen of ground glass near the back of the camera. The eye of an 

 observer in rear of the camera (and not nearer than eight inches from 

 the ground glass,) sees this inverted image distinctly depicted upon 

 the ground glass. 



(8.) If the ground glass is removed, an aerial image is seen to 

 occupy the position from which the ground glass was just removed. 



The dioptric media of the eye are made up of the cornea, aqueous 

 humor, crystalline lens, and vitreous humor, all differing in density 

 and consequently in their refractive power, but the effects produced 

 by their combination resemble those produced by a double convex 

 lens, or a single spherical refracting surface, having its convexity 

 towards the less refracting medium. Like a double convex lens, it 

 too has an optical centre, any ray passing through which is either un- 

 refracted or refracted parallel to its original direction, thus : — 



Let E (Fig.. I.) represent a section of an eye, and'C its optical 

 centre ; any ray as iVE passing through C will pass on to the retina 

 unrefracted, or at least parallel to its original direction. 



The position of the optical centre varies according to the focal 

 adjustment of the eye, being further from the retina, when the eye is 

 adjusted for near objects, than it is when adjusted for distant ob- 

 ject?. 



