186 RECENT PROGRESS OF THE THEORY OF VISION. 



reduced to those elements alone which are absolutely necessary 

 for exact vision. Fig. 29, from Henle, shows a thin transverse 

 section of this central depression made on a retina which had 

 been hardened in alcohol. Lh (Lamina kyalina, membrana 

 limitang) is an elastic membrane which divides the retina from 

 the vitreous. The bulbs (seen at b) are here smaller than else- 

 where, measuring only the 400th part of a millimetre in dia- 

 meter, and form a close and regular mosaic. The other, more 

 or less opaque, elements of the retina are seen to be wanting, 

 except the corpuscles (<?), which belong to the cones. At f are 

 seen the fibres which unite these with the rest of the retina. 

 This consists of a layer of fibres of the optic nerve (n) in front, 

 and two layers of nerve cells (gli and gle), known as the internal 

 and external ganglion layers, with a stratum of fine granules 

 (gri) between them. All these parts of the retina are absent at 

 the bottom of the fovea centralis, and their gradual thinning 

 away at its borders is seen in the diagram. Nor do the blood 

 vessels of the retina enter the fovea, but end in a circle of 

 delicate capillaries around it. 



This fovea, or pit of the retina, is of great importance for 

 vision, since it is the spot where the most exact discrimination 

 of distances is made. The cones are here packed most closely 

 together, and receive light which has not been impeded by other 

 semi-transparent parts of the retina. We may assume that a 

 single nervous fibril runs from each of these cones through the 

 trunk of the optic nerve to the brain, without touching its 

 neighbours, and there produces its special impression, so that 

 the excitation of each individual cone will produce a distinct 

 and separate effect upon the sense. 



The production of optical images in a camera obscura depends 

 on the well-known fact that the rays of light which come off 

 from an illuminated object are so broken or refracted in passing 

 through the lenses of the instrument, that they follow new 

 directions which bring them all to a single point, the focus, at 

 the back of the camera. A common burning glass has the 

 same property ; if we allow the rays of the sun to pass through 



