OBJECTIONS TO THE USE OF COLOURED GLASSES. 55 



thickness. It is easy to conceive, that although in the solar spectrum the yellow ray 

 is vastly more luminous than the blue, if glasses or coloured media were used in experi- 

 ments of any kind, results of precisely an opposite nature might be obtained. We can 

 imagine a piece of yellow glass so thick as actually to transmit less light than a thinner 

 piece of the blue. With glasses and absorbent media, therefore, not only must the na- 

 ture of transmitted light be discovered, but thickness must also be taken into account. 

 234. Suppose, as an illustration, plants were made to grow, or to decompose car- 

 bonic acid, under a thin piece of blue cobalt glass, such as that of which finger-glasses are 

 made, and the result compared with what was occurring with another set of plants in 

 action under a thick or deep yellow glass. It might turn out that the former would 

 yegetate more vigorously, and fix more carbon, and produce more chlorophyl than the 

 latter, because more light actually fell upon them, though the medium through which it 

 came was blue. We must not forget that the mode of action in producing chemical 

 decomposition, and the mode of action in producing vision through such an optical 

 contrivance as an eye, proceed upon different principles. In the action of the eye 

 time does not enter as an element (Ap., 536). In the decompositions produced by ra- 

 diant matter, or by light, it does, A faintly luminous object does not become brighter 

 and brighter as we continue to look steadfastly at it ; it has assumed its maximum of 

 brilliancy at the first glance. But a faintly luminous beam, falling upon leaves of 

 plants, or upon any changeable compound, continues to produce an increasing effect 

 as time passes on. In our experiment, more and more carbonic acid is decomposed, 

 and more and more oxygen set free as the time is prolonged. And so when absorbent 

 media, as stained glass, are used, the final effect is dependant on the total amount of 

 light that has been furnished ; and hence the turbidity, or thickness, or partial opacity 

 of those media must be taken into account, as much as their colour-giving relations. 

 In the prismatic experiment this source of disturbance does not occur. 



CHAPTER VII. 



ON THE VARIOUS IMPONDERABLE AGENTS EXISTING IN THE DIFFERENT REGIONS OF 



THE SPECTRUM. 



CONTENTS : Different Agents existing in the Spectrum. Description of the Tithonic 

 Rays. Their Name. Physical Independence of Heat. Of the Chemical Rays. 

 Thfir Constant Association with Light. Detithonizing Action of Yellow 'Solutions. 

 Argument for their Independence. Other Invisible Principles in the Sunbeam, 

 such as the Phosphoric Rays. Examination of the Theory of M. Becquerel. 



235. FROM the preceding chapter, we have arrived at the conclusion that the impon- 

 derable principle which directs the digestion of plants is found at a maximum in the 

 yellow space of the spectrum, or accompanies yellow light 



236. But it has been stated (224) that there are no less than three other principles 



I 



