450 SPECIAL PHYSIOLOGY. 



tute a ganglion capable of perceiving light, and the fibres of the optic 

 nerve merely communicate these impressions to the sensorium." 



According to either description, the essential point maintained by 

 Draper is, that the sentient or receiving part of the retina, is the pos- 

 terior; that which is in contact with the black pigment or receiving 

 screen, whence the impression is taken. In accordance with this ex- 

 planation, the same thing occurs, though with infinitely more delicacy, 

 in the eye, as when we pass the tip of the finger over the surfaces of 

 bodies, and recognize on them warm and cold spaces. " The club- 

 shaped particles of Jacob's membrane, are truly tactile organs, which 

 communicate to the surface of the retina the conditions of temperature 

 of the black pigment." This is confirmed by the fact that, at the 

 point where the optic nerve enters the eye, and where only the fibrous 

 elements of the retina are present, there is no vision. 



Moreover, according to Dr. Draper, "If no other argument was ad- 

 duced for departing from the opinion usually expressed, which attrib- 

 utes this function to the retina, the thickness of that structure would 

 be sufficient ; images can only form with precision or sharpness upon 

 an abrupt surface. And since it is now indisputably ascertained that 

 both the chemical effect and heating effect of the rays of light depend 

 on their absorption, those effects being in direct proportion to the com- 

 pleteness with which absorption is taking place, we are justified in in- 

 ferring that, since the eye is sensible to rays of so low a degree of 

 intensity, and to each of the colored ones equally, its screen of recep- 

 tion must not only be a superficies, but likewise a black one. Such a 

 surface the black pigment is. In the case of Albinos, and animals in 

 which the black pigment is imperfectly developed, the receiving sur- 

 face or screen is still the interior of the choroid. Under such circum- 

 stances vision must be indistinct." 



It accords also with facts in the diffuse sensibility of some of the 

 lower orders of animals to light, and in the structure of the ocelli, 

 u to consider that the primary effect of rays of light upon the black 

 pigment is to raise its temperature, and this to a degree in relation to 

 their intensity and intrinsic color; light which is of a yellow tint ex- 

 citing most energetic action, and rays corresponding to the extreme 

 red and the extreme violet the feeblest. The varied images of external 

 objects which are thus painted upon the black pigment, raise its tem- 

 perature in becoming extinguished, and that in the order of their bril- 

 liancy and color; the pigment thus discharging a double duty, as a 

 surface of extreme sensibility for calorific impressions, and also as 

 darkening the interior of the globe." 



Again, it is necessary, in order that a perfect image may be formed, 

 that the luminous body should possess an inherent degree of heat of at 

 least 1000, for rays from a source having a lower temperature, can- 

 not pass through a stratum of water or the humors of the eye. The 

 eye is different from the thermometer in this, that while the latter is 

 sensible to rays of every sort, the eye is not; the necessary presence 

 of absorbent media stopping all rays of low refrangibility. 



Professor Draper, carrying out, through a series of years, experi- 

 ments instituted by Count Rum ford many years ago, has concluded 



