vii EETINAL EXCITATION 355 



varying in number, which are characteristic of the different 

 chemical elements. 



The colours of the various bodies that do not give out light 

 depend on their capacity for reflecting if they are opaque, or 

 refracting if transparent, certain of the light-rays, and of absorbing 

 all others. Their colours are saturated in proportion to the 

 quantity of rays of any given wave-length which they reflect or 

 refract; brighter, in proportion to the total of visible rays which 

 they reflect and refract darker, according to the number of 

 visible rays they absorb. 



V. In order that the radiations of the ether shall be able to 

 excite the retina, it is essential, not only that their wave-length 

 on which their visibility depends shall fall between red and violet 

 of the spectrum, but that two other conditions shall be fulfilled, 

 namely, they must have a certain intensity and a certain duration. 



The intensity of a luminous sensation depends on the amplitude 

 or kinetic energy of the ether vibrations that excite it, and on the 

 degree of retinal excitability at the moment of stimulation. 



The threshold stimulation, i.e., the minimum of luminous 

 intensity essential to vision, is usually very low. By a method 

 of great delicacy Aubert found that the normal eye is capable of 

 perceiving light one million times weaker than ordinary daylight. 

 Other conditions being equal the threshold of retinal excitability 

 also varies within physiological limits in different subjects. Astro- 

 nomers know, in fact, that many people are able to see certain 

 stars which are invisible to others apart from any ametropia of 

 the eye. 



The normal eye is capable of distinguishing which is the 

 brighter or stronger and which the duller or weaker of two 

 luminous sensations. The minimal objective difference in in- 

 tensity of two lights which the eye is capable of recognising 

 is known as the threshold of difference. Visual acuity, or the 

 power of distinguishing between the intensity of two lights, is 

 greater in proportion as the value of the differential fraction is 

 less. Innumerable careful researches have proved that the differen- 

 tial fraction is not a constant, as required by Weber's law, but 

 varies considerably according to the absolute magnitude of the 

 luminous stimulus. 



The, typographical tallies used for determining visual amity are modelled 

 upon those of Snellen, and are so constructed that if the subject has normal 

 .-ight, i.e. visual acuity = 1 (V=l), he is aide to n-ad the smallest letters of tin- 

 scale at a distance of :> m., and tin- ollin>, which are increasingly larger, at 

 .on-espondiiig distances of 7'50-10-15-20-30- lo-.">o metres. The. visual acuity 

 is indicated liy a fraction, which is obtained by taking as numerator the 

 figure that indicates the di.-tance at which the, letten of the table 'MIL be read, 

 and as denominator the figure th.it indie ite> the distance at which the 

 smallest letters of the table which the subject is able to recognise can be read 

 by a normal eye. If he can only recognise at a distance of 5 m. the letters 



