'BKOWNIAN' MOVEMENT 431 



the objective is withdrawn ; whilst those which are of less refractive 

 power have a centre which is whiter and smaller, and a black ring 

 which is broader and darker when the objective is lowered. 



Monochromatic light. The same phenomena are observed by 

 yellow monochromatic light, except that the diffraction fringes are 

 more distinct, further apart, and in greater numbers than with 

 ordinary light. 



A fat-globule, indeed, seems to be composed of a series of con- 

 centric layers like a grain of starch. With blue light these fringes 

 are also multiplied, but are closer together and finer, so that they 

 are not so easily visible. 



Yellow monochromatic light, therefore, constitutes a good 

 means for determining whether "the striae seen on an object are 

 peculiar to it or are only diffraction lines. In the former case 

 they are not exaggerated by monochromatic light ; but if, on the 

 contrary, they are found to be doubled or quadrupled with this 

 light, we may be certain that they are diffraction fringes. 



But there is no source of fallacy, to a certain class of workers, so 

 much to be guarded against as that arising from errors in the inter- 

 pretation concerning movements as such, and especially concerning 

 the movement exhibited by certain very minute particles of matter in 

 a state of suspension in fluids. The movement was first observed in 

 the fine granular particles which exist in great abundance in the 

 contents of pollen grains of plants known as ihefovilla, and which 

 are set free by crushing the pollen. It was first supposed that they 

 indicated some special vital movement analogous to the motion of 

 the spermatozoa of animals. But it was discovered in 1827, by Dr. 

 Robert Brown, that inorganic substances in a state of fine trituration 

 would give the same result ; and it is now known that all substances 

 in a sufficiently fine state of powder are affected in the same manner, 

 one of the most remarkable being the movement visible in the con- 

 tents of the fluid cavities in quartz in the oldest rocks. These have 

 probably retained their dancing motion for reons. A good illustra- 

 tion is gamboge, which can be easily rubbed from a water-colour 

 cake upon a glass slip and covered, and will at once show the 

 characteristic movement ; so will carmine, indigo, and other similarly 

 light bodies. But the metals which are from seven to twenty times 

 as heavy as water require to be reduced to a state of minuteness 

 many times greater ; but, triturated finely enough, these also show 

 the movement, for a long time known, from the name of its dis- 

 coverer, as Brownidn movement, but now more generally called 

 pedesis. , 



The movement is chiefly of an oscillatory nature, but the particles 

 also rotate backwards and forwards on their axes, and gradually (if 

 persistently watched) change their places in the field of view. It is an 

 extremely characteristic movement, and could not be mistaken for 

 any vital motion by an observer acquainted with both; but the 

 student must familiarise himself with this kind of motion or he will 

 be utterly unable to distinguish certain kinds of motion in minute 

 living forms in certain stages of their life from this movement, and 

 will make erroneous inferences. 



