TRANSACTIONS OP THE SECTIONS. 15 



On the Geometric Forms, and Laws of Illumination of the Spaces which 

 receive the Solar Rays, transmitted through Quadrangular Apertures. By 

 Sir David Brewster. 



He said his attention was called to this subject by an accidental discussion on the 

 point, whether or not Aristotle, in explaining the circularity of images formed by 

 quadrilateral apertures, employed the appropriate idea when he said that those images 

 were, to a certain extent, quadrilateral, but appeared circular, from the eye being unable 

 to recognise faint impressions of light. Prof. Whewell, in his ' History of the Philo- 

 sophy of the Inductive Sciences,' had distinctly stated, that Aristotle had not used the 

 appropriate idea, and that the question was entirely a geometrical one, the appropriate 

 idea being the rectilinear nature of light. Having been led accidentally to consider 

 the subject, he (Sir D. Brewster) had determined in a simple manner the form of the 

 aperture at all distances, and had been led to take the same view of the subject with 

 Aristotle, who seemed to have employed the appropriate idea. 



On Luminous Lines in certain Flames corresponding to the defective Lines in 

 the Sun's Light. JBy Sir David Brewster. 



After noticing Fraunhofer's beautiful discovery as to the phsenomena of the line D 

 in the prismatic spectrum, Sir David said, he had received from the establishment of 

 that eminent man, at Munich, a splendidprism, made for the British Association, and 

 one of the largest, perhaps, ever made ; and, upon examining by it the spectrum of 

 deflagrating nitre, he was surprised to find the red ray, discovered by Mr. Fox Talbot, 

 accompanied by several other rays, and that this extreme red ray occupied the exact 

 place of the line A in Fraunhofer's spectrum, and equally surprised to see a luminous 

 line corresponding with the line B of Fraunhofer. In fact, all the black lines of Fraun- 

 hofer were depicted in the spectrum in brilliant red light. The lines A and B turned 

 out in the spectrum of deflagrating nitre to be both double lines ; and, upon examining 

 a solar spectrum under favourable circumstances, he found bands corresponding to these 

 double lines. He had looked with great anxiety to see if there was anything analogous 

 in other flames, and it would appear that this was a property which belonged to almost 

 every flame. 



On the Structure of a Part of the Solar Spectrum hitherto unexamined. 

 By Sir David Brewster. 



He had, by means of the prism from Munich, been enabled to extend the solar 

 spectrum beyond the point where, according to Fraunhofer, it terminated immediately 

 at the side of the line A, and he (Sir David) found one part to consist of about sixteen 

 lines, placed so near to each other, that it was difficult to recognise the separation ; 

 but the lines, as they approached to A, were much nearer to each other than as they 

 receded from it ; consequently, that portion of the spectrum appeared concave, resem- 

 bling so much the scooped-out lines of a moulding on wood, that it was scarcely pos- 

 sible to suppose that the beholder was not looking at such a moulding. He was led to 

 observe an analogous structure near the line B ; and upon carrying on this comparison 

 of structure of one part of the spectrum with that of another, it seemed to him, that, 

 by and by, something important would result ; for there was a repetition of a group 

 of lines, and similar lines, through different parts of the spectrum, as if the same cause 

 which produced them in one part produced them in another. 



On the Luminous Bands in the Spectra of various Flames. 

 By Sir David Brewster. 



He had endeavoured to procure all the minerals and artificial salts and other sub- 

 stances capable of combustion which could be had ; and, in order to have a suitable 

 combination, he used an oxygen light analogous to the Bude light. Every one con- 

 ducting these experiments was aware that it was necessary to pass the light through a 

 narrow aperture; but this would reduce the intensity of the light so much, as to make 

 it difficult to observe the rays at the extremity of the spectrum ; but he found that he 

 could obtain the effect of a small aperture, by merely inclining the prism ; so that, with 

 a good prism, the great lines ia the solar spectrum might be seen by using an aperture 



