576 PRINCIPLES OF GENERAL PHYSIOLOGY 



the reader is referred to the books by Rutherford (1913) and by Soddy (1911, 

 1914), and, as regards its action on living tissues, to that by Finzi (1914). 



X-RAYS 



The X-rays of Riintgen, in their effect on cells, are also similar to those of 

 ultra-violet light, but the destructive effect continues for a long time and does 

 not appear at once in its full magnitude. The results of repeated exposure are 

 very serious, since the rays penetrate to deep tissues and are able to produce 

 degeneration of the seminiferous cells of the testis, for example. This fact of 

 relatively slight absorption by tissues, other than bone or certain metallic salts, 

 has made the use of X-rays very valuable in discovering the nature of displacement 

 of bone, and so on. We have also seen their application to the study of the 

 movements of the alimentary canal. 



As to the nature of X-rays, the view is now generally held that they are 

 similar to light waves, electro-magnetic, but of a very short wave length, O'l to 

 10 *b 



Further information may be obtained from the book by Kaye (1914). 



PHOTOGRAPHY IN PHYSIOLOGY 



The use of photographic methods of recording phenomena has been described 

 above (page 460). We may add here the photography of absorption spectra. 

 It is plain that plates sensitive to the whole of the visible spectrum are 

 necessary. Wratten's " Panchromatic " will be found suitable, especially in the 

 fine-grained variety known as " M " plates. These are prepared especially for 

 the photography of objects under the microscope. The kind of negative usually 

 required is not identical with that of landscape photography ; for convenience of 

 reproduction, a " hard negative " generally serves best. The developer used by 

 Willstatter and Stoll (1913) will be found excellent for such purposes. It is made 

 thus : Solution I., 500 c.c. distilled water, 50 g. crystallised sodium sulphite, 

 5 g. hydroquinone, 1 g. metol. Solution II., 500 c.c. distilled water, 50 g. 

 potassium carbonate. For use, mix 30 c.c. of each with 60 c.c. of water (120 c.c. 

 together) and develop for three minutes at 18 to 20 in darkness and fix in acid 

 bath. 



The methods of direct colour photography, such as Lumi^re's " autochrome " 

 process, have been used to prepare some beautiful photographs of stained 

 microscopic preparations. 



SUMMARY 



The dependence of life on the receipt of radiant energy from the sun makes it 

 of importance to understand how this energy can be converted into forms useful 

 to the organism without the necessity of passing through the state of heat, in 

 which a considerable part of the free energy would be lost. 



The manner in which this is done is by conversion directly into chemical 

 energy by means of what are known as photo-chemical reactions. 



These reactions are also of importance and interest with relation to the 

 receptor organs for light impressions. 



Light cannot act unless it is absorbed (Grotthus's law). The amount absorbed 

 by a medium of various thicknesses is regulated by Lambert's law, which states 

 that it is a logarithmic function of the thickness. 



The " extinction coefficient " is the most useful form in which the absorption 

 power of a particular solution is expressed. The extinction coefficient is the 

 negative logarithm of the light which has passed unabsorbed through a thickness 

 of 1 cm. It is different for different wave lengths. This is the form in 

 which it is most convenient for practical use, but the original definition made it 



