10 



PRINCIPLES OF GENERAL PHYSIOLOGY 



violet light when impinging on various substances is observed by the micros, ope. The colour 

 of the light emitted by these fluorescent substances differs according to their composition, so 

 that it may be possible to detect the presence in the living cell of substances otherwise 

 invisible. Further information will be found in Chapter XIX., and in the papers by 

 Stiibel (1911), and by Heimstadt (1911). 



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1 and -1. Dividing nuclei from (fill plate of Salamander larva. Unstained, in glycerol. Photo- 

 graphed with ultra-violet light of 280 >XM- The chromatic substance appears as if stained. 



3. Kdge of sternal cartilage of Newt. Living. Photographed with ultra-violet light. The 



nuclei arc opaque. 



4. The same. Photographed with ordinary light. The nuclei are transparent. 



.1. Red blood corpuscles of the Newt. Living. Photographed with ordinary light. Although 

 oblii|ue illumination was used, the nuclei are almost invisible. Traces of diffraction are 

 seen around the corpuscles. 



. Amoeba. Living. Photographed with ordinary light. The nucleus is just visible, but 

 transparent. 



(Nos. 1 to 3 after Kohler.) 



INTRA-VITAL STj 



tNG 



The second method, that of staining the living cell, has been of much service. 

 Ehrlich (1886) was the first to show that methylene blue stains living nervous 

 structures. 



A simple way of observing this fact is given by Michaelis (1902, p. 99). A short piece of 

 the intestine of a mouse is placed for about half an hour to one hour in a solution of methylene 



