and the Function of Chlorophyll in Plants. 63 



liquids, they evidently grow in relative obscurity in compari- 

 son with their normal conditions, even in relation to the 

 colour the action of which is wished to be investigated ; hence 

 the results thus obtained correspond only to the actions pro- 

 duced in plants by insufficient intensities of light. Moreover 

 the function of chlorophyll itself contributes to the weakening 

 of the result. I mention this because certain conjectures 

 respecting the function of chlorophyll, which have since been 

 verified, first induced me to take up these experiments with 

 higher intensities of light. 



So long as I employed only comparatively inconsiderable 

 augmentations of the intensity, I obtained no decisive results. 

 I at last attained satisfactory effects when I ventured to bring 

 organic forms, vegetable and animal cells and tissues, into 

 the plane of an image of the sun which I projected in the 

 focus of an achromatic lens of 60 millims. diameter. 



The apprehension which perhaps at first arises, that organic 

 structures must under these circumstances be forthwith de- 

 stroyed by the thermal action of the solar image, is, as a 

 closer consideration and direct experiment show, unfounded. 

 With proper precautions, the object can be observed undis- 

 turbed for a considerable time in the sun's image, as indeed is 

 approximately shown by the phenomena in the so-called solar 

 microscope. In this way the influence of the radiation upon 

 an entire tissue and upon each single cell, nay, even upon the 

 different form-constituents of a single cell, can be separately 

 studied, and with a little attention the thermal and photo- 

 chemical effects of the radiation can be certainly and sharply 

 distinguished. 



Hence this method of microscopical photochemistry (as I 

 would call it) is preeminently adapted for investigating whether 

 any, and what, photochemical actions of light take place in 

 protoplasm and in the formed constituents of the cell-body ; 

 and it is equally suitable for determining the relative degree 

 of diathermasy of the cell-contents and the cell-membrane. 

 In this way also the effects of higher degrees of heat can be 

 more conveniently brought into view than by aid of heated 

 object-tables. Lastly, it is self-evident that the method is 

 applicable for animals and animal tissues as well as for plants ; 

 and with it we can at the same time demonstrate the sensa- 

 tion of heat in the lowest classes of animals (Protozoa and 

 Coelenterata) , and in certain cases ascertain the truth respec- 

 ting the presence, and the seat, of the perception of light. 



The experiments in reference to this which, in the course 

 of my investigations, I have made on animals I will commu- 

 nicate subsequently ; I will here preliminarily enter more 



