A NEW OPHTHALMOSCOPE. 87 



inches in diameter, and having a focus of 2^ inches. In the corres- 

 ponding position of the tube B, or close to the plate glass reflector, 

 the lena H is placed convexo-plaue, of 5 inch focal length : If inches 

 from this is another lens also convexo-plane, and having a focal 

 length of 5 inches, and having the same diameter, viz : li inches. 



THE CAMEKA. 



The camera consists of a mahogany box 3 inches square and 7 

 inches high, having (to secure steadiness) a base 6 inches square. 



At the aperture in the centre of the anterior side there is a brass 

 collar fitted, through vrhich slides the tube .containing the lenses. 

 At the opposite side of the camera is a central aperture 2| inches 

 square, behind which is a slide with a piece of ground glass 2^ inches 

 square. This slide moves in grooves for the purpose, and can be re- 

 moved to make way for a elide containing a sensitized plate also 

 about 2^ inches square. 



PHOTOGRAPHING. 



As yet I have not attempted a photograph of the retina of the hu- 

 man eye, but have confined mj experiments to the lower animals, and 

 I have used solar light only in order to shorten the time as much as 

 possible, but I do not doubt that diffused light, particularly that re- 

 flected from a bright cloud, would, with a longer •' exposure," an- 

 swer very well. In using the instrument for this purpose, a table of 

 the ordinary height is placed near a window where the light of th@ 

 sun falls upon it. It is well to have the shutters closed, and a 

 beam of solar light admitted of the size of the illuminating tube, but 

 this is not absolutely necessary, if precautions are taken to prevent 

 difiused light entering the camera, and the ground glass is shaded 

 while examining the image on its surface. 



The camera is turned at right angles to the source of light, and the 

 tube A or illuminating tube turned so that the light falls full into 

 the tube, and is incident upon the whole of the lens Gr. 



When the camera and tube are in proper position, a cone of light 

 issues from the end of the camera tube through the centre of the 

 aperture in the diaphragm, which is the condensed light from the 

 lens Gr, reflected from the plate glass D. This cone forms a focus 

 about ^ inch outside the diaphragm which can be seen by holding a 

 thin piece of white paper near the diaphragm. In photographing 



