672 THE POPULAR SCIENCE MONTHLY. 



it would be necessary to have the source of illumination a mere point. 

 Of course the effect of such abrupt transitions from perfect illumina- 

 tion to total darkness, if it were the plan of Nature, would be most in- 

 convenient and painful, if not destructive to the eyes. We are pro- 

 tected from this by the phenomena of double shadows, and the grad- 

 ual passage from darkness to light, although each luminous ray moves 

 undeviatingly in its straight line. Light cannot turn corners like 

 sound. This is explained by the excessive shortness of the lumi- 

 nous as compared with sonorous waves. Sound-waves are so large 

 that they flow around objects in the air, and consequently cast but 

 feeble shadows, although Deschanel (from whose admirable work on 

 " Natural Philosophy " our cuts are borrowed) states that Colloclon, 

 in his experiments on the transmission of sound through the water of 

 the Lake of Geneva, established the presence of a very sharply-de- 

 fined sound-shadow in the water behind the end of a projecting wall. 

 It is necessary to say, however, that the foregoing statement that 

 light cannot turn corners is only true in the common and general ex- 

 perience of it. If we carry our experiments with light down to 

 so fine a point that we reach the dimensions of its waves, it is then 

 found that they are capable of bending round obstacles. If sunlight be 

 allowed to pass through an exceedingly fine slit, and then to fall on a 

 screen of white paper, colored bands appear, called diffraction fringes; 

 that is, the white light, in its passage through the minute opening, has 

 been interfered with and broken up into its component colors. We 

 have here, however, a new order of effects which will require to be 

 separately considered. 





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VIVISECTION. 



By MICHAEL FOSTEE, M. D., F. E. S. 



PROFESSOR OF PHYSIOLOGY IN THE UNIVERSITY OF CAMBRIDGE. 



IN the following pages I propose to inquire whether it is desirable 

 that physiologists should continue the practice of what is common- 

 ly called vivisection, to which they have hitherto been accustomed. By 

 vivisection I understand the operating with cutting instruments or by 

 other means on the still living bodies of animals. The word " living " 

 requires, perhaps, some further definition. In the long series of 

 changes through which the body of a living animal passes, from full 

 functional activity to complete decomposition, there are three chief 

 stages, each of which may be arbitrarily taken as the end of life. 

 There is the time at which consciousness is lost, the time at which the 

 breath stops and the heart ceases to beat, and the time at which the 

 muscles become rigid with the death-stiffening. The succession of the 

 three events is always in the same order, but the interval of time be- 



