s MICRO-ORGANISMS AND FERMENTATION. 



the photographic plate is sensitive to certain chemically 

 active rays of light which cannot be detected by the human 

 eye, so that photography can reproduce certain characters 

 of the preparation which would be entirely missed by direct 

 observation. It may be anticipated that the new method of 

 photographing by ultra-violet light will bring about notable 

 advances, for it will thus be possible to distinguish a greater 

 number of fine characteristics and preparations otherwise 

 colourless will appear to be composed of differently coloured 

 parts. 



The ultra-microscope that has just been applied to technical 

 purposes makes it possible to distinguish small details that 

 are invisible with all earlier optical devices. Its design is 

 based upon a special method of lighting, discovered by Sieden- 

 topf and Zsigmondy, whereby the preparation is brightly 

 illuminated with rays of light, falling in a direction at right 

 angles to the axis of the microscope. The illumination is 

 so arranged that a single layer of the preparation is lit up 

 whilst the remainder is in shadow. With the help of this 

 apparatus particles can be distinguished of a size of four- 



millionths of a millimetre. (The symbol fj. is used for mm. 



a AyV/WV/ 



and ju/u for - mm.) The effect of such an illumination 



may be realised by recalling the well-known appearance of 

 particles of dust floating in a beam of sunlight penetrating 

 into a darkened room, when the particles are viewed in a 

 direction approximately at right angles to the beam of light. 



A micro-chemical examination is of value for ascertaining 

 the composition of the individual parts of a micro-organism. 

 After treatment of the cells with reagents, the reaction brought 

 about, and especially the colour reaction, may be studied 

 under the microscope, with a view of determining the chemical 

 composition of the part in question. As an example the proof 

 may be cited that yeast cells, in the later stages of develop- 

 ment, contain the reserve food stuff, glycogen (a carbo-hydrate). 

 In order to detect this substance in the cell an iodine solution 

 is added to the preparation (2 grammes iodine, 6 grammes 

 potassium iodide, and 120 c.c. of water). The albuminoid 



