USE OF THE SPECTROSCOPE IN STUDY OF PLANT LIFE. 87 



was to try to test his conclusions, using, however, coloured glasses 

 instead of fluids ; and the results I have obtained all tend to corroborate 

 his. 



It is impossible to ascertain absolutely the amount of water trans- 

 pired which is due to any particular band of colour in the solar spec- 

 trum ; for the difficulties are insuperable. To say nothing of other 

 influences at work to aid in the elimination of water, such as external 

 and internal variations of temperature, dryness of the air, and, above 

 all, variations in the intensity of sunlight, it is impossible to get glasses 

 monochromatically pure in any colour excepting red. Hence the results 

 cannot be more than relative or approximately true. Such as they are, 

 however, I find, like Wiesnee, that the largest amount of water trans- 

 pired is coincident with those parts of the spectroscope wherein lie 

 the strongest absorption-bands of chlorophyll. 



Character of the Coloured Glasses. — Before giving the results 

 of my experiments, it will be desirable ta describe the character of 

 the glasses employed. 



Red. — This is a pure monochromatic ruby-red, which transmits no 

 other rays whatever besides red light ; and it is that portion of the 

 spectrum which contains the very strong chlorophyllian absorption- 

 bands Nos. I. and IT. The thickness of the glass is one-tenth of an 

 inch. 



Yellow. — This glass passes not only the yellow, but all the red 

 and green rays as well, up to F in the blue; but stops the rest of the 

 most refrangible half of the spectrum. As this glass not only transmits 

 yellow and green which contain the fainter chlorophyllian bands 11. , 

 III., and IV., but the powerful one No. I. as well, one would, on 

 d priori grounds, infer that more, and not less, transpiration would 

 occur with yellow glass than with red ; but such is not usually the case. 

 Moreover, the loss of water under ordinary clear colourless glass (one- 

 twelfth of an inch in thickness) is sometimes less than under either 

 red or violet glasses alone ; so that from these facts one is led to infer 

 that the presence of the brightest or yellow rays is an actual impedi- 

 ment to transpiration, or in some way hinders the action of the red 

 and violet. The thickness of the yellow glass used is one- twelfth of 

 an inch. 



Green. — This excludes red and violet rays, but transmits light 

 which includes the position of the chlorophyllian bands III., IV., V., 

 and VI. It is one-twelfth of an inch in thickness. 



Blue. — This has the red end greatly subdued, appearing quite black 

 in diffused light; but in direct sunlight a broad black band is seen in 

 the midst of a small quantity of dull red. It includes green; but a 

 black band in the position of the chlorophyllian band No. IV. Hence 

 the glass transmits light which would include the bands No. V., VI., 

 and VII. The thickness of the glass is one-tenth of an inch. 



Violet. — This passes less green, but rather more red, than the blue 

 glass. The light transmitted would include bands Nos. II., V., VI., 

 and VII. A dark absorption-band occurs about the position of No. IV. 



