APPARATUS OF VEGETABLE PHYSIOLOGY. 261 



great care, and involves the use of a good deal of appa- 

 ratus. The same results may be more easily arrived at by 

 means of these two flasks which I have on the table, and 

 which were devised by Prof. Sachs, of the University of 

 Wurzburg (No. 3944). They are simply large glass bottles 

 with double walls, the space between the walls of the one 

 being filled with an ammoniacal solution of copper oxide, 

 and of the other, with a solution of potassium bichromate 

 the former being of a blue, the latter of an orange colour. 

 In this way we are enabled to split up the solar spectrum 

 into two halves, the one half of its rays (including part 

 of the green, the blue, indigo, violet, and the actinic rays) 

 penetrates into the cavity of the blue flask the other 

 half (including part of the green, the yellow, orange, 

 red, and dark rays) reaches the interior of the yellow flask. 

 That the properties of these two sets of rays are very 

 different is indicated by the fact that a piece of sensitive 

 paper, such as is used in photography, rapidly becomes 

 darkened when exposed in the blue flask, whereas it is 

 scarcely affected by exposure in the yellow one ; and we 

 may therefore conclude that, in the former case, we have 

 to deal with rays of considerable actinic power, whereas, in 

 the latter, the rays are comparatively inert. Let us see 

 how these two sets of rays affect the process of growth. 

 Here are some young pea-plants which have been grown for 

 some days inside these flasks, and have been supplied 

 equally with air and moisture. It will strike you at once 

 the growth has taken place much more rapidly under the 

 influence of yellow than under that of blue light, for there 

 is a difference of several inches between the heights of the 

 plants. From this we may conclude that the rays of high 

 refrangibility are more active in arresting growth than 

 those of lower refrangibility. 



A careful examination of these two plants will show 

 that they differ not only in the amount, but also in the 

 direction of their growth. You see that this plant grown 

 in the blue flask has become considerably curved in a 

 definite direction, and this was towards the source of its 

 illumination, whereas the plant grown in the yellow flask 

 is nearly straight. This tendency to bend towards the 

 source of light is due, then, to the action of the highly 

 refrangible rays upon the growing cells. These rays have 



