( 147 ) 



is given to (lie 11^^ or Tl^ liulil ; ;il lliosc |)l;u'('s the image of the Sun, 

 |)li()logra|)Iie(l in Iivdi'ogeii lines, must tlieretbre he leys dai-k. 



The nither faint charaeter of the hydrogen tlocenli, the absence 

 of sharp outlines and of strong contrasts in the structural elements, 

 we ascribe to the dispersion bands of hvdrogen being relatively 

 narrow and so allowing rays with a great variety of refractive indices 

 to pass simultaneously through the second slit of the spectrohelio- 

 graph. The hydrogen pliotogra[)hs too would show liner details, like 

 those in Kj light, if the dispersion of the apj)aratus were still greater 

 and the second slit still narrower. 



We believe that we have shown that every peculiarity, thus far 

 noticed in the photographs obtained with the spectroheliograph, may 

 easily be deduced from the same fiuidamcntal hypothesis regarding 

 the constitution of the Sun, which lias already proved capable of 

 giving a coherent interpretation of the solar phenomena known 

 before. Not a single new hypothesis w^as required. 



Physiology. — ''On nrtific'ml (ind imtiwal iiercc-stuitulation dud ihc 

 qiuuiüt[i of ('iiei'<jij i/iro/rcd.'" By Prof. H. Zwaardemakek. 



A living nerve, laid bare, can be stimulated artificially in a nund)er 

 of ways; there are but two kinds of stimuli, however, the effect of 

 which is instantaneous and whose strength can be accurately regulated, 

 namely mechanical and electrical stinudi. Mechanical stinnilation has 

 l)een considerably improved l\v an artifice of Schafere), who used 

 falling drops of mercury instead of electrically driven little hammers. 

 When using droplets the size of which is about equal to the breadth of 

 the nerve, even with a very small height of fall distinct effects we 

 obtain, manifesting themselves by contraction of the muscle which 

 has remained connected with the nerve. Scuafek himself obtained 

 this result with a drop weighing 100 mgr. falling through 10 mm. 

 In our lal)oratory his method gaxe still l)etter results; an effect was 

 noticed already with a drop of 50 mgr. and a height of fall of 5 mm. 

 Such a dro[i possesses at the end of its fall an energy of 24.5 ergs. 

 Not the energy as such is a measure of the stinuilus, however. 

 Apparently the energy has also in this respect to be considered 

 as consisting of aji intensity-factor and a capacity-factor '^), and this 



1) Proc. Physiol. Soc. 20 Jan. iliUl. 



2) W. OsTWALD. Ber. d. k. Sachs. Ges. d. Wissenschalicii 18".)2, matli. physik. 

 Gl. S. ^215. 



G. Helm. Die Energetik in ilner gescliiclitl. Enlwickluug 1898 S. 277. 



10* 



