SCIOPTICONMANUAL. 11 



SIZE ON THE RETINA. 



A A (Fig. 2) represent* the visible portion of the 

 sclerotica of the human eye, which is a camera obscura 

 in perfection. 1 I represents 

 the iris (or window shutter), 

 in the middle of which is the 

 pupil (or aperture). As the 

 retina is only about half an 

 inch behind the optical centre, 

 it follows that the images of 

 distant objects upon it must be xfy- 9. 



very minute. 



For example, the figure of a man 6 feet high, seen at 

 a distance of 40 feet, produces an image upon the retina 

 the height of which is about y^th part of an inch. The 

 face of such an image is included in a circle whose 

 diameter is about y^th of the height, and therefore 

 occupies on the retina a circle whose diameter is about 

 the -j-j^th part of an inch ; nevertheless within this 

 circle, the eyes, nose, and lineaments are distinctly seen. 

 The diameter of the eye is about T ^th of that of the face, 

 and therefore, though distinctly seen, does not occupy 

 upon the retina a space exceeding ^(j^iuirD^^ f a 

 square inch. How infinitely delicate must be the 

 structure of the retina or canvas on which this exquisite 

 miniature is delineated to receive and transmit details 

 so minute with such marvellous precision ! 



SIZE IN THE: PHOTOGRAPHIC CAMERA. 



A man G feet high, standing for his picture 10 feet 

 from a camera tube whose lenses require the screen of 

 ground-glass adjusted to 5 inches, gives a 3 inch picture. 

 For we have (in inches) 



120 : 5 : : 72 : x hence x = * = 3. 



