32 



THE POPULAR SCIENCE MONTHLY. 



structure of the brain, as is the secretion from the structure of a 

 gland. 



Wherever the material conditions of mental activity in the shape 

 of a nervous system are lacking, as is the case with plants, the scien- 

 tist cannot admit the existence of soul-life ; and here he but seldom 

 finds his views controverted. But what answer is to be made him if 

 he were to require, as the condition of his believing in a soul of the 

 universe, that there should be shown to him somewhere in the world, 

 bedded in neuroglia, and nourished with warm arterial blood under 

 due pressure, a system of ganglia and nerves corresponding in ex- 

 tent to the mental power of such a soul ? 



Finally, the question arises whether the two limits of our knowl- 

 edge of Nature are not perhaps identical, i. e., whether, supposing 

 we understood the nature of matter and force, we should not also 

 understand how the substance that underlies them could, under certain 

 conditions, feel, desire, and think. Certainly this is the simplest hy- 

 pothesis, and, according to well-known principles of scientific research, 

 until it is disproved it must be preferred to that other hypothesis, 

 which, as we have said, makes the universe doubly incomprehensible. 

 But such is the nature of things that we cannot attain clearness of 

 view with regard to this point, and it were idle to dwell upon it. 



With regard to the enigma of the physical world the investigator 

 of Nature has long been wont to utter his " Ignoramus " with manly 

 resignation. As he looks back on the victorious career over which he 

 has passed, he is upheld by the quiet consciousness that wherein he 

 now is ignorant, he may at least under certain conditions be enlight- 

 ened, and that he yet will know. Bnt as regards the enigma what 

 matter and force are, and how they are to be conceived, he must resign 

 himself once for all to the far more difficult confession — 

 "Ignoeabimtjs ! " 



THE CEOOKED COUESES OF LIGHT. 



AN article in the April Mo]!?thly explained the formation of lumi- 

 nous images upon the principle that light moves in straight 

 lines through any uniform transparent medium ; but at the same time 

 no agency in Nature illustrates in so many ways its capability of 

 being turned from a direct course. It may be thrown back by sur- 

 faces either directly in its own path, or at all possible angles, and it 

 may be warped out of its course in various degrees as it passes through 

 bodies, although in all cases the change of direction is governed by 

 inflexible laws. The throwing back of rays from surfaces is known as 

 the reflection of light ; the bending or the fracture of the ray as it 

 traverses a body is called the refraction of light. 



