NATUIAL PIIILOSOrilY. 179 



intensity of the candle's light at the distance of a mile is less than a 20,- 

 000,000 of its intensity at th distance of a foot : hence the energy 

 which renders the candle perfectly visible a mile off would have to be 

 multiplied by 1,300 X 20,000,000, or by 26,000,000,000, to bring it up 

 to the intensity of that powerless radiation which the eye receives from 

 the electric light at a good distance. Nothing, I think, could more for- 

 cibly illustrate the special relationship which subsists between the optic 

 nerve and the oscillating periods of luminous bodies. The nerve, like a 

 musical string, responds to the periods with which it is in accordance, 

 while it refuses to be excited by others of vastly greater energy, which 

 are not in unison with its own." 



Opacity and Transparency. What is the physical meaning of opac- 

 ity and transparency as regards light and radiant heat ? The luminous 

 rays of the spectrum differ from the non-luminous ones, simply in period. 

 The sensation of light is excited by waves of ether, shorter and more 

 quickly recurrent than those which fall beyond the extreme red. But 

 why should iodine stop the former, and allow the latter to pass ? The 

 answer to this question no doubt is, that the intercepted waves are those 

 whose periods of recurrence coincide with the periods of oscillation possi- 

 ble to the atoms of the dissolved iodine. The elastic forces which sep- 

 arated these atoms are such as to compel them to vibrate in definite pe- 

 riods ; and, when these periods synchronize with those of the etheral 

 waves, the latter are absorbed. Briefly defined, then, transparency in 

 liquids as well as in gases, is synonymous with discord, while opacity is 

 synonymous with accord between the periods of the waves of ether and 

 those of the molecules of the body on which they impinge. All ordina- 

 ry transparent and colorless substances owe their transparency to the dis- 

 cord which exists between the oscillating periods of their molecules, and 

 those of the waves of the whole visible spectrum. The general discord 

 of the vibrating periods of the molecules of compound bodies with the 

 light-giving waves of the spectrum may be inferred from the prevalence 

 of the property of transparency in compounds, while their greater har- 

 mony with the extra-red periods, is to be inferred from their opacity to 

 the extra-red rays. Water illustrates this transparency and opacity in 

 the most striking manner. It is highly transparent to the luminous rays, 

 which demonstrates the incompetency of its molecules to oscillate in the 

 periods which excite vision. It is as highly opaque to the extra-red un- 

 dulations, which proves the synchronism of its periods with those of the 

 longer waves." Prof. Tyndall. 



THE PHYSICAL ASPECT OF THE SUN. 



In a paper on the above subject, read before the British Associa- 

 tion at its last meeting, the author stated that he had recently, with 

 improved instruments, taken many occasions of scrutinizing the aspect 

 of the sun^s disc in regard to spots, faculaa, and the general porosity 

 of the surface. For tracing the path of a spot across the disc a 

 Kilner eye-piece was employed with five engraved transit lines the 

 intervals being equal to 10 in the central part of the sun's circumfer- 

 ence. In drawing, negative eye-pieces of the ordinary kind were 

 sometimes employed; at others, a peculiar kind arranged by himself, 

 with powers varying from 75 to 300 ; the best performances being 

 usually between 100 and 200; the higher powers, however, being oc- 



