2 5 6 



LECTURE XIII. 



a relatively pure spectrum produced by means of a microspectroscope, he 

 ascertained that the Bacteria collected especially in the region between 

 the lines B and C, that is, at the junction of the orange and red, and that 

 there was a secondary maximum of aggregation in the blue just beyond 

 the line F : these regions coincide with the most marked absorption- 

 bands of the chlorophyll-spectrum. Inasmuch as the collection of the 

 Bacteria in these regions is an indication that oxygen is being evolved 

 there, these observations afford most valuable support to the view that 

 those rays are the most active in the decomposition of carbon dioxide 

 which correspond in position to the most conspicuous absorption-bands 

 in the chlorophyll-spectrum. 



a B C 



FIG. 35 (after Engelmann). A portion of a Cladophora-filament seen under the 

 microscope in the solar spectrum, the chlorophyll-corpuscles being omitted 

 from the drawing. The principal lines of the spectrum are indicated. The 

 aggregation of the motile Bacteria about the filament is seen to be greatest 

 between the lines B and C. 



It has been mentioned (p. 152) that Engelmann is of opinion that 

 some of the colouring-matters which are characteristic of certain Algae 

 (phycoxanthin, phycocyanin, and phycoerythrin) have the same func- 

 tion as chlorophyll. He obtained with various brown, bluish-green, 

 and red Algae, results similar to those given above with reference to 

 chlorophyll, though the points of maximum aggregation were not the 

 same inasmuch as the absorption- spectrum of these colouring-matters is 

 not the same as that of chlorophyll. Thus in the case of blue-green 

 Algae the maximum of aggregation is riot in the red-orange, but in the 

 yellow, and in the case of red Algae, in the green. 



Engelmann was enabled to estimate quantitatively the relative activity 

 of the different rays of the spectrum by the following method. He 

 observed the moment when, in each successive region of the spectrum, 

 the movement of the Bacteria in the neighbourhood of the Alga began, 

 and then ascertained the width of the slit. The width of the slit is a 

 measure of the intensity of the light ; therefore the wider the slit the 



