662 SUMMARY OF CURRENT RESEARCHES RELATING TO 



spectrum, it is advisable to have several different objectives. The 

 sharpness of the spectra is such that some hundreds of Fraunhofer 

 lines can be made perfectly visible. The luminous intensity, even 

 when an ordinary gas-flame is used, is sufficient with a slit of 

 0-01 mm. wide to observe bacteria under a high power. By replacing 

 the prism by a grating, the apparatus may also be adapted in a simple 

 manner to the formation of a microscopic interference spectrum. The 

 results now communicated were obtained by means of the prismatic 

 microspectrum. The first question examined was that of the relative 

 extent of the disengagement of oxygen by the green cells in the 

 different regions of the spectrum. In this examination two methods 

 can be adopted, viz. — 1. The method of simultaneous observation. 

 2. The method of successive observation. 



Simultaneous Method. — This consists in observing simultaneously 

 the action of the different rays of the spectrum on different juxtaposed 

 points of the same object. The object should have a regular struc- 

 ture. Conferva, Oscillarice, and some diatoms, are especially suitable. 

 The object is placed in a transverse position in the microspectrum, 

 that is, perpendicularly to the direction of the Fraunhofer lines. 

 The following are the phenomena then observed. 



When the luminous intensity increases, starting from zero, the 

 bacteria in repose in the immediate neighbourhood of the green cells 

 begin to move first of all in the red, most frequently between B and 

 C, and nearer to the latter. Increasing the intensity of the illumina- 

 tion, the action extends on both sides to the commencement of the 

 ultra-red and as far as the violet. The accumulation of the bacteria 

 and the rapidity of their movement, remain in the beginning at their 

 maximum in the red. With green cells (Euglena, CEdogonium, Cla- 

 dophora), but not with brown (diatoms) and blue-green (Oscillariese), 

 there appears in sunlight (not in gaslight) a minimum in the green 

 about E and a second maximum about F. When the bacteria are 

 numerous, we see in such cases a kind of graphic representation of 

 the relation between the wave-length and the assimilative energy, in 

 which the abscissae are represented by the object and the ordinates 

 by the respective depths of the layer of bacteria. 



In the case of very great luminous intensity the differences are 

 reduced, because then the accumulation and the rapidity of movement 

 become very great at all points. 



If, starting from the maximum, the luminous intensity decreases 

 gradually, the different aspects just described are reproduced in 

 inverse order. 



Successive Method. — In this method the object (preferably very 

 thin) is placed successively in different parts of the spectrum, deter- 

 mining each time the narrowest width of the slit at which the bacteria 

 begin to move. The results confirm in general those of the first, as 

 is shown by two tables given by the author. 



In one respect, viz. the situation of the maximum, the results of 

 Encelmann differ very remarkably from those hitherto obtained by 

 macroscopic methods by the best observers (Draper, Sachs, Pfeffer). 

 These authors attribute to the yellow rays the strongest assimilative 



