l8g6-97-[ THE FUNCTION OF INDIRECT VISION. 309 



central retina for the perception of forms and colours, and on the other 

 the superiority of the eccentric retina in sensitiveness to light and to 

 changes in space. These supplement each other to make the eye what 

 it is, and injury to either is equally harmful to the efficiency of the 

 sense of sight. The peripheral retina must be more sensitive to light, 

 and if we prevent it from exercising this function we will injure the 

 motory mechanism in the most vital way. Such injuries will be induced 

 by all those artificial optical arrangements which give advantages of 

 any kind to direct vision at the expense of the light intensity offered to, 

 indirect vision. All kinds of smoked and blue or otherwise coloured 

 glasses which absorb the rays destined for direct and indirect vision in 

 unequal ratio, must be classed as harmful arrangements of this character 

 This is especially the case with concave glasses which are smoked or 

 coloured throughout. The rays which are transmitted through the 

 centre of these glasses to the central regions of the retina suffer less 

 absorption than those rays which, coming from the side, have to pass 

 through a considerably greater extent of absorbing medium. The 

 indirect vision in this case, is, therefore, at a disadvantage, and its 

 movement-inducing function will be greatly damaged, for the peripheral 

 parts do not get the necessary amount of light to give the characteristic 

 impulses for directing the eye towards the objects concerned. There- 

 fore concave glasses which are coloured throughout should NEVER BE 

 U.SED, aud the prescription and sale of them should be prohibited. If, 

 however, correction of the refractive state of the eye, and protection 

 against high intensities are required at the same time, the two conditions 

 should either be met by separate glasses, or, if this be impracticable, the 

 glasses should not be coloured throughout, but should be composed 

 (after the manner of achromatic lenses), of a perfectly colourless concave 

 part of the proper refraction power, and a coloured part with parallel 

 surfaces, having the refractive power zero. 



