1877.] On some Phenomena connected with Vision. 487 



do not succumb quite so rapidly. The cause of this deadly influence 

 exerted by fresh water has been found to depend on the absence of the 

 mineral constituents of sea-water, and not, as Agassiz also supposed, 

 on the difference of density between the former and the latter. Chloride 

 of sodium alone., dissolved in appropriate amount in fresh water, deprives 

 the latter, to a great extent, of its deleterious influence ; but this is not 

 the case with any other substance I have tried. Brine acts as an 

 anaesthetic, and, in respect of depriving the tentacles and manubrium of 

 their muscular tonus, exerts an influence the opposite of that which is 

 exerted by fresh water. 



II. " On some Plienomena connected with Vision. By B. 

 Thompson Lowne, F.B.C.S.^ Arris and Gale Lecturer on 

 Anatomy and Physiology at the Royal College of Surgeons, 

 Lecturer on Physiology at the Middlesex Hospital Medical 

 School. Communicated by Prof. Stokes^, Sec. R.S. Be- 

 ceived Septemher 25_, 1876. 



1. On the Physiological Effect of Ruled Surfaces. 



2. On the Time required to Produce or Obliterate a Retinal Image. 



3. On the Relation of the foregoing Observations to Fechner's law. 



1, On tlie Physiological Effect of Ruled Surfaces. 



Some months ago it occurred to me that an investigation of the rela- 

 tion of the shades produced by ruled surfaces (as in engravings) with 

 those produced by variations in the intensity of illumination (shadows) 

 would a:fford useful data in connexion with the physiological action of 

 light upon the retina. 



On examining a number of woodcuts and hne-engravings I found that 

 there are usually about nine different shades, which may be expressed by 

 the following fractions, which represent the ratio of black to white upon 

 the surface : — 



012345678 

 8' 8' 8' 8' 8' 8' 8' 8' 8* 



Such engravings, when seen at such a distance that every line makes a 

 distinct picture and can be separately perceived, exhibits a sufficiently 

 gradual series of tints or shades passing regularly from white to black. 

 In this case I assume that the intensity of the sensations produced by a 

 given surface is directly as the number of retinal elements stimulated, 

 so long as the nature and intensity of the illumination remain constant. 



I next endeavoured to determine the effect of variations in the intensity 

 of the stimulus in the following manner : — 



I repeated Lambert's well-known experiment, in which two candles 

 are placed at distances D and from a screen, an opaque body being 

 interposed so that each candle casts a shadow upon the screen, the light 



