8i2 THE POPULAR SCIENCE MONTHLY. 



angular position of the prism, with regard to the collimator, is a matter 

 of importance, the distinctness and purity of the spectrum depending 

 in a great measure on that position. Perhaps as good a way as any is 

 to find by actual trial the angular position of the prism which gives 

 the best results, thus : Focus the telescope for distant vision, align 

 the telescope with the collimator, and move the slit till it (the slit) is 

 seen distinctly in the telescope ; then put the prism in place approxi- 

 mately, and move it round its axis until a position is found where 

 (supposing the light of the sun or diffused daylight to be under exam- 

 ination) the lines in the green part of the spectrum are seen at their 

 best advantage. It must be remembered that a slight change of focus 

 is necessary for every color. When the bounding edges of the ribbon 

 of variegated light which forms the spectrum are seen sharply defined 

 at the same time that the lines across the spectrum are distinct, the 

 adjustment is pretty correct ; when such is not the case, the slit and 

 lens are not in their proper relative positions. The narrower the slit, 

 consistent with the necessary supply of light, the finer and more dis- 

 tinct are the lines. A black cloth, or a pasteboard box, with suitable 

 apertures for the collimator and telescope tubes, should be placed over 

 the prism to shut out all extraneous light. In conclusion, I must 

 remark that I do not pretend to describe an instrument capable of 

 doing exact or delicate work. My aim is simply to show with how 

 little trouble or ingenuity a spectroscope may be put together which 

 will help materially those students who wish to obtain a good general 

 idea of a branch of science which has done more to unravel the mys- 

 teries of nature than any other inquiry with which the human mind 

 has ever been occupied. 



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THE SOUKCE OF MUSCULAR POWER. 



By H. P. AEMSBY. 



THE question of the source of muscular power is essentially a ques- 

 tion concerning transformation of energy. The most character- 

 istic distinction between plants and animals is, that the former appro- 

 priate force from outside themselves, from sunlight, and store it up as 

 potential energy in the various complex compounds which they form 

 in ; while animals draw their supplies of force entirely from those 

 compounds in which it has been stored up by plants, and from which 

 it is set free again when they are decomposed in the organism. 



In a word, the plant converts the actual energy of the sunlight into 

 the potential energy of organic compounds, the animal converts the 

 potential energy of the organic compounds into actual energy, w r hich 

 manifests itself as heat, motion, electricity, etc. ; in the plant the 

 spring is coiled up, in the animal it uncoils, exerting an amount of 



