144 ANNUAL OF SCIENTIFIC DISCOVERT. 



is bounded at the violet end by a visible beam of definite refrac- 

 tion. This inference is a physical absurdity, since it presupposes 

 the existence of bodies, which, under any arbitrary angle of in- 

 cidence, totally reflect a visible beam, or completely absorb it on 

 the surface. The dispersion-formula derived from the imperfect 

 series, even if correct for large wave-lengths, can offer no ex- 

 planation as to what really occurs at that^ limit at the more re- 

 frangible end of the spectrum. 



"While this limit, with substances of weak dispersion, would 

 lie very far in the ultra-violet, it sometimes appeared in the green 

 in the bodies which I investigated. With no single body of this 

 group were even traces of interference observed in the violet. 

 The reason of this phenomenon might be sought in a strong reflec- 

 tion of these rays at the surface, or in a strong absorption in the 

 interior. It has been shown that the latter is the preponderating 

 cause of the absence of interference-bands. Then they always 

 vanish gradually with increasing thickness, from the violet to the 

 red end of the spectrum, and are very soon only present in the 

 yellow and red. Hence tlie absorption increases with decreasing 

 ivave-lengths, and, indeed, continuously so for a certain position in the 

 spectrum, which is special to each substance, and so quickly that on 

 the other side of it no ray can pass through a layer of the thick- 

 ness of half a wave-length. 



"Hence, in transmitted light sufficiently thick layers of bodies of 

 pre-eminent dispersion always appear yellow-red or red. I have 

 sought in vain for a substance of this kind which would be trans- 

 parent with green, blue, or violet light. 



" To meet any objections to these matters of fact arising from 

 the mention of apparent exceptions, I must make the following 

 remarks : 



" Thin kiyers can be prepared in different ways, which strongly 

 absorb the light, and are transparent to other than yellow or red 

 light; such layers, however, like glass coated with soot, are not 

 to be regarded as bodies, but as loosely connected apparatus of 

 individual particles, and can only be quoted as exceptions if it 

 be proved generally that they possess refractive and dispersive 

 properties. For example : let chlorine, bromine, iodine, sul- 

 phur-vapor, or sulphuretted hydrogen act on thin layers of 

 silver ; then thin layers of chloride, bromide, iodide, or sulphide 

 of silver are formed, which, in comparison with the metals and 

 metallic oxides described, are very transparent, and show in 

 the spectroscope beautiful interference-bands. If, however, the 

 intensity or duration of the action of these agents exceeds a cer- 

 tain limit, the structure of the layers is destroyed ; the same are 

 then to be regarded as aggregates of many particles (in several 

 cases microscopic crystals), although they appear to the eye as 

 coherent masses ; they are more opaque than the metal itself, 

 and show no trace of interference-bands in the spectroscope." 

 Philosophical Magazine. 



