224 Dr. Alex. Hodgkinson on 



such investigation was to be conducted, the probability is 

 that in their essential features such results would be almost 

 identical. 



Besides these spectra produced by the radiation of 

 special kinds of light, another class of spectra exists to 

 which the name of absorption spectra has been given, and 

 since these constitute the subject of the present communica- 

 tion I shall, before considering the proposed method for 

 their investigation, briefly allude to their nature and mode 

 of formation. 



Firstly, as regards absorption generally. All bodies are 

 now recognised as being more or less pervious to light ; and 

 no substance is known which is absolutely transparent. 

 Even the metals, regarded as the most opaque of bodies, 

 when reduced to films of sufficient tenuity, become trans- 

 parent to certain rays of light only, and, therefore, when seen 

 by transmitted light appear coloured. Thus, gold appears 

 greenish-blue ; silver, blue ; mercury, violet-blue ; and so on. 

 As such films become thinner, however, they transmit more 

 and more light, until, as in the case of this gold film 

 (exhibited) the transmitted light is almost colourless. Films 

 produced in this way are, according to Faraday, " scarcely 

 the Tooth, perhaps not the Tooth, of a wave undulation of 

 light" in thickness. The reason why a body appears 

 coloured when illuminated by white light is because certain 

 rays have become absorbed, and the transmitted light, being 

 deficient in these rays, is robbed of one of the essential 

 elements for the formation of white light, namely a suitable 

 admixture of variously coloured rays. When such coloured 

 light is examined with the spectroscope, and the constituent 

 rays thus spread out so as each to occupy its special posi- 

 tion in the spectrum, the locality of the missing rays is 

 indicated by a dark space, due to a deficiency of light at the 

 spot. Such dark space constitutes a so-called absorption 

 band, and a spectrum containing one or more of such 



