1865.] Physics. 689 



VIII. PHYSICS. 



Light. — With few exceptions spectroscopes and spectrum observations 

 constitute the principal novelties in this branch of science, during the 

 last few months. Foremost among the latter may be mentioned a 

 lecture by Mr. Huggins, delivered before the Koyal Institution, on the 

 physical and chemical constitution of the fixed stars and nebulae. 

 The speaker stated that spectra might be divided into three orders, 

 viz. : — 



1. A continuous spectrum, unbroken by bright or dark lines, 

 which indicates that the light has not undergone any modification on 

 its way to us. Also, that its source is an opaque body, almost 

 certainly in the solid or liquid state. Such a spectrum gives no 

 information of the chemical nature of the substance from which the 

 light emanates. 



2. A spectrum of bright lines separated by dark spaces ; this 

 informs us that the source of the light is matter in the gaseous state. 

 By a comparison of the bright lines of such a spectrum with the lines 

 of terrestrial flames we may discover whether any of these terrestrial 

 substances exist in the distant and unknown source of light. The 

 spectra of many of the nebulae are of this order. 



3. A continuous spectrum interrupted by dark lines ; this shows 

 that the light has passed through vapours which have deprived it of 

 certain refrangibilities by a power of selective absorption. 



Since Kirchhoff has shown that these dark lines agree exactly in 

 position with the bright lines which the vapours would emit if in a 

 luminous state, a comparison of these dark lines with the bright lines 

 of terrestrial vapours will indicate whether any of these are present 

 in the vapours through which the light has passed. The spectra of 

 the fixed stars are of this order. In the case of these bodies, the 

 vapours that produce the dark lines immediately surround them, and 

 are those of the substances of their surfaces. The chemical constitu- 

 tion of the stellar atmospheres will correspond, at least in part, with 

 that of the stars themselves. 



What are the stars? Endeavour with the most powerful tele- 

 scopes to approach them, still they assume no apparent size ; they 

 remain under the highest magnifying powers what they appear to the 

 unaided eye — diskless, brilliant points. 



Until quite recently, our knowledge of the stars might be summed 

 up thus : — That they shine ; that they are immensely distant ; that 

 the motions of some of them show them to be composed of matter 

 endowed with a power of mutual attraction. 



Since these bodies are self-luminous, we may hope to gain by 

 prismatic analysis more information of their nature, than it is possible 

 to do of the planets, which all shine by reflecting the sun's light. 



The results on the light of the stars Aldebaran and a Ononis 

 (Betelgeux) are given in the following table : — 



