VISION. 379 



to the wants of the animal. We consider it as affording a series of il- 

 lustrative instances of the use of means for the production of definite 

 ends, and therefore as exhibiting the evidences of design ; but what are 

 we to make of the other, the parallel, the individual case, in which, in 

 succession, the organ presents each one of these particular forms, and in 

 which not one is ever used, save only the most complete, the last? 

 There would seem here to be little of adaptation, little of means to an 

 end, nothing of design. But does not this prove to us (and the other 

 organs of sense will furnish a similar argument) that the development of 

 the various animal tribes, and of each individual of those tribes, takes 

 place under the operation of a far-reaching and common law, and that the 

 particular condition which any species presents, or even any individual 

 at some special period of his existence, is a manifestation of the degree 

 or extent to which that law has been carried out ? 



CHAPTER XX. 



OF VISION. 



Analogy between Sound and Light. Comparative Anatomy of Vision. Perception of Warmth. 

 Structure of Ocelli. Use of Lenses. Physical Principle of the Organ of Vision. 



Description of the Human Eye. Optical Action of its Parts. Spherical and Chromatic Aberra- 

 tion: Receiving Screen of the Eye is the black Pigment. Long and short Sight, and their 

 Correction. Limits of Vision are included in one Octave. Limit in estimating the Brightness 

 of Light. 



Nervous Mechanism of the Eye : its Structure and Functions. Manner of Perception by the 

 Retina. The black Pigment absorbs the Rays. Single and double Vision. Duration of Im- 

 pressions. Ocular Spectra. Erect Vision. Idea of the Solidity of Bodies. Hypothesis of 

 the Action of the Retina. 



Accessory Apparatus of the Eye. The Eyebroios. Eyelids. Lachrymal Apparatus. Muscles 

 of the Ball. 



THE physical difference between waves of sound and waves of light 

 has been already stated to consist in this, that the vibrations . 

 which give rise to the former coincide with the direction in tween sound 

 which the wave passes, but those of the latter are transverse. and llght ' 

 There is, however, notwithstanding this difference, a general analogy be- 

 tween the structure of the ear and that of the eye, and provision must 

 be made in the case of the organ of sight for determining the fundamental 

 peculiarities which are necessary to be determined in the case of the or- 

 gan of sound ; that is to say, intensity, wave length, or time of vibra- 

 tion, and, in addition thereto, the requirements of this particular prob- 

 lem demand that means should be included for ascertaining with exact- 

 ness the shape of objects and their relative positions. 



