230 



tion of the omitted point increases or diminishes the probability of 

 guilt, according as a + b is greater or less than 2 a, or a greater or 

 less than b : That is to say, the preponderance of the greater proba- 

 bility is diminished by the considei'ation of each unproved note. If, 

 as was supposed to be generally the case in criminal trials, the pro- 

 bability from the evidence preponderates against the accused, then 

 he does get a benefit from the due consideration of each unproved 

 note, though very far from the benefit he would have gained by prov- 

 ing it in his favour. 



2. On the Gradual Production of Luminous Impressions on 

 the Eye, and other phenomena of Vision. By William 

 Swan, F.R.S.E. 



The object of this communication was to ascertain the relation be- 

 tween the apparent brightness of a light, and the time during which 

 it acts on the eye. In order to examine the intensity of luminous 

 impressions of short duration, the author made use of discs, having 

 sectors of known angles cut out of their circumferences, which were 

 made to revolve at known velocities between the eye and a luminous 

 object. In this manner, the object is seen at each revolution of the 

 disc for a short interval of time, of which the duration is easily as- 

 certained. An instrument termed a selaometer (from eiXas, bright- 

 ness'), to indicate its use as a measure of the intensity of luminous 

 impressions, was devised for the purpose of comparing the brightness 

 of the flashes caused by the revolution of the disc, with a light of 

 known intensity. This instrument consists of two screens, placed so 

 as to face each other, having each a circular aperture of the same 

 diameter, to which is fitted a piece of obscured glass. A disc, hav- 

 ing a sector of a known angle, I'evolves in front of one of these 

 screens, so that the aperture in it is visible at each revolution 

 of the disc throughout the sector. The apertures are illuminated 

 by gas flames behind them, which admit of having their distances 

 from the screens varied, so as to increase or diminish the illumination 

 of the apertures. A rectangular prism of glass is placed half way 

 between the apertures, with its faces inclined at angles of 45° to the 

 lino joining their centres ; so that they are seen in apparent contact 

 by reflexion from the faces of the prism, and their i-elative brightness 



