380 PERCEPTIONS OF WARMTH. 



Of the different methods which we might follow in the discussion of 

 Importance of the mechanism of the eye, none, perhaps, is more satisfacto- 

 the compara- Qr j ea( j s to clearer conclusions, than that which is pre- 



tive anatomy J ' ' 



of vision. sented by comparative anatomy. It is just as difficult to 

 take a complicated organ, such as the eye of man, and from the study 

 of it to deduce the significance of its various parts, as it would be to take 

 a complicated human contrivance and determine from it the properties of 

 its mechanical elements. It is scarcely from the watch or other delicate 

 machine that we should expect to make plain the properties of the lever 

 or the wheel, and experience shows that it is only by the attentive study 

 of the cases presented by comparative physiology those experiments 

 made for us by nature, as CUVIER has called them that we can hope to 

 advance to the perfect solution of this problem. 



Treating the subject, therefore, in this way, we observe that, in the an- 

 Confused er * ma ^ ser ^ es ' ^ on S Before an y thing like a distinct organ of vis- 

 ception of ion can be detected, there is yet a perception of light and 

 warmth. darkness. The hydra, a fresh-water polype, offers an exam- 

 ple, for this animal seeks the sunny side of the vessel in which it is 

 placed, preferring it to the shade. In the absence of every vestige of a 

 visual organ, there can not be a doubt that its 'movements depend on the 

 perception of warmth, just as when a man who is totally blind passes 

 from the sun into the shade, his feelings at once notify him of the change. 

 In a physiological sense, it is of no interest to us to inquire into the phys- 

 ical nature of this effect, whether light is identical with heat, or whether, 

 when light falls upon a body, it turns into heat. We have only to ac- 

 cept it as a fact capable of abundant experimental proof, that, whenever 

 rays of light fall on a surface, that surface becomes warm. This, as we 

 shall now find, is the key of all the explanations we have to give. 



Dr. Franklin made an experiment to the following effect. He placed 

 Dr. Franklin's on the snow, on a sunshiny winter day, pieces of cloth of dif- 

 experiment. ferent colors black, yellow, white, etc., etc. in such a posi- 

 tion that the sun's rays fell equally on them. After a certain length of 

 time, on examining them, he found that the black cloth had melted its 

 way deeply into the snow, the yellow to a less depth, and the white 

 scarcely at all. He therefore drew the conclusion that, when they are 

 receiving light, surfaces become warm in proportion to the depth of their 

 tint, and that, of all surfaces, one having a velvety blackness is most sen- 

 sitive, because it can exert the most powerful absorbent agency. 



On this principle seem to be constructed the ocelli of the lower tribes. 

 Ocelli of lower These consist of a collection of pigment granules, usually 

 animals con- O f a re( j bi ac k or <j ar k co lor, seated on the expansion of a 



structedon . n . 



Franklin's nervous thread. The principle which is clearly contained in 

 principle. ^ s mechanism j s that of relieving the general surface from 



