362 VISION. LECT. XX. 



image, but its light will be proportionately fainter: this, 

 perhaps, is the reason that, in nature there is no apparatus 

 for vision analogous to the camera obscura. 



2. The Mosaic Dioptric Instrument. Miiller has described 

 a very curious arrangement found in the eyes of some insects. 

 Imagine that in front of the retina, and perpendicularly to 

 its surface, there is placed an immense number of small 

 cones filled with a transparent matter, and whose sides are 

 invested by a black and opaque pigment, capable of ab- 

 sorbing all the rays which do not traverse the cone in a 

 direction parallel to its axis. A transparent and convex 

 membrane forms the external surface, which is also the base 

 of the cone. In the summit or apex of the cone is fixed 

 the extremity of the nervous fibre, which, according to 

 Wagner, is prolonged into the interior of the cones, on 

 whose sides it is expanded. It is easy to conceive how 

 distinct vision may be obtained by means of this arrange- 

 ment. Of all the luminous rays which emanate in every 

 direction from an object, and fall upon every point of the 

 surface of the eye, those only which proceed from a deter- 

 minate point of the object, and which traverse one of the 

 cones parallel to its axis, can reach the retina. The dis- 

 tinctness of the image will depend, then, on the number of 

 cones arranged on the nervous surface ; whilst the intensity 

 of the image must always diminish with their number. We 

 can also understand how, with this apparatus, the extent of 

 the field of vision may be increased, by augmenting the 

 convexity of the spherical segment representing the eye. 



3. The Concentrating Dioptric Instrument. Lastly, to 

 obtain an apparatus of vision still more perfect than the one 

 just described, it was necessary so to construct it that the 

 distinctness of the image should not be obtained at the ex- 

 pense of its brightness; and, that rays which fall very 

 obliquely on the surface of the eye should be made to con- 



