jBh' Mechanical Science. ' 457 



8. Duration of the Effects of Light upon the Eye.^U. Plateau 

 of Liege has endeavoured to determine the length of time during 

 which the impression of certain luminous rays upon the eyes 

 remains ; and has given the following results : 



Flame 0''.242 



Ignited Charcoal 0".229 



White , 0".182 



Blue !•'.',;;... 0M86 



Yellow ,..t. ::.::.. omts 



Red ..;;.r;i.r:--.. o'M84 



■ ■• . » 



9. On the Measurement, of the Tjitensity of Light, by M. Peclet.— 

 A very usual photometrical process is to interpose an opaque body 

 between a white screen and the two lights to be measured, and 

 to move the latter until the shadows produced are of equal inten- 

 sity ; the intensity of the lights being then as the square of their 

 distances from the shadows they illuminate. Sometimes a trans- 

 lucent body, as unpolished glass or oiled paper, is used in place 

 of an opaque one, the shades produced by transmission being 

 observed. 



In both these methods, the apparent intensity of the shadow 

 varies with the position of the observer. If the shadows are equal 

 when observed from a point perpendicular to the white screen at 

 the middle of the distance of the two shades, they will be no longer 

 so on removing from that position, and the shadow nearest to the 

 observer will always appear the darkest. These apparent variations 

 are greater as the shadows are farther apart, or with reflected sha- 

 dows as the screen is smoother, or with transmitted shadows as the 

 interposed obstacle is more diaphanous. 



The explanation given of this fact is, that unpolished opaque 

 bodies, like paper, plaster, &c. never disperse the light incident 

 upon them, in an uniform manner, more rays passing in the direc- 

 tion in which regular reflexion would take place, than in any other. 

 Hence, when two equal shadows are produced upon such a surface, 

 either by two equal lights at equal distances, or by two unequal 

 lights at unequal distances ; the shadow nearest to the observer 

 must necessarily appear deeper than the other, because it is en- 

 lightened by the nearest light, the rays from which are reflected in 

 greatest abundance away from the observer ; and, on the contrary, 

 the shadow further from the observer should appear lightest, be- 

 cause the rays which fall on it from the furthest light are reflected 

 in greatest abundance towards the side on which the observer 

 stands. The reason, also, why the effect is greater as the shadows 

 are further apart is evident ; and why in every case it is reduced to 

 nothing when the observer is in a plane perpendicular to the screen 

 and equidistant from the two shadows. 



From these facts and explanations it may be concluded, that, in 

 all photometrical measurements by reflected shadows, the screens 

 should have all smoothness removed from them, and the two sha- 



