2o6 Dk. a. Hodgkixson on 



the abo\'e optical phenomena, and therefore to belong to 

 neither microscopic nor ultra-microscopic group as defined 

 above. Films of mica may be separated so thin as to be in- 

 capable of reflecting light of any colour at any incidence, and 

 therefore to appear black under any conditions of illumina- 

 tion. The same condition is met with in the case of the 

 thin film constituting the central spot of Newton's rings. 

 True it is, that in these instances, the invisibility is ascribed 

 to interference, arising, as pointed out by Young, from the 

 loss of half an undulation which occurs when light is 

 reflected at the surface of the denser of two media. Still, 

 even though this loss of half an undulation were an un- 

 doubted truth, the fact remains that transparent films, the 

 thickness of which is less than a quarter of a wave-length of 

 violet light, neither reflect light nor give any other positive 

 optical evidence of their existence. In the case of inter- 

 ference from thicker films,on the other hand, we havereflected 

 and refracted colours of the most varied description. Again, a 

 complex arrangement of portionsof such invisiblefilms would 

 still remain invisible, and the same is true of structures 

 generally when composed of elements too thin to produce 

 optical effect. Thus, in the case of mica, certain crystals of 

 chlorate of potash, and other minerals which exhibit cleavage, 

 we notice no internal evidence of arrangement in lamella;, 

 and yet no one can doubt that such structural arrangement 

 does exist, but the lamella; being in optical contact, that is 

 separated by intervals of less than a quarter of a wave- 

 length, and themselves of similar dimensions, fail to give 

 optical evidence of their existence, and thus the mass appears 

 homogeneous. Just as transparent films when of a certain 

 thinness are invisible, so must transparent particles when of 

 the same diameter be invisible, and a body composed of 

 such small particles would appear homogeneous ; and just 

 as a thin invisible film which gradually increases in thick- 

 ness when illuminated by white light, first reflects those rays 



