688 COLOR-VISION AND COLOR-BLINDNEES. 



only, a region which the fibers avoid by curving round it, and in which 

 the other hiyers of the retina are much thinner than ehsewhere, so as 

 to leave a depression, and are stained of a lemon-yellow color. In a 

 zone immediately around this yellow spot each cone is surrounded by 

 a single circle of rods ; and as we proceed outwards towards the 

 periphery of the retina, the circle of rods around each cone becomes 

 successively double, triple, quadruple, or even more numerous. The 

 yellow spot receives the image of the object to which the eye ia actually 

 directed, while the images of surrounding objects fall upon zones which 

 surround the yellow spot; and the result of this arrangement is that 

 generally speaking, the distinctness of vision diminishes in i)ropoition 

 to the distance of the image of the object from the retinal center. The 

 consequent eft'ect hns been well described by saying that what we see 

 resembles a picture, the central part of which is exquisitely finished, 

 while the parts around the center are only roughly sketched in. We 

 are conscious that these outer parts are there ; but if we desire to see 

 them accurately, they must be made the objects of direct vision in their 

 turn. 



The indistinctness with which we see lateral objects is so completely 

 neutralized by the quick mobility of the eyes, and by the manner in 

 which they range almost unconsciously over the whole field of vision, 

 that it seldom or never forces itself ui)on the attention. It may be 

 conveniently displayed by means of an instrument called a perimeter, 

 which enables the observer to look steadily at a central spot, while a 

 second spot, or other object, is moved along an arc, in any meridian, 

 from the circumference of the field of view towards the center, or vice 

 versa. Slight ditierences will be found between individuals ; but, si)eak- 

 ing generally, a capital letter one-third of an inch high, which is legi- 

 ble by direct vision at a distance of 16 feet, and is recognizable as a 

 dark object at 40° or 5()o from the fixing point, will not become legible 

 at a distance of 1 foot, until it arrives within about 10<^. 



The image formed upon the retina is rendered visible by two diflereut 

 conditions, — that is to say, by diflerences in the an)Ount of hght which 

 enters into the formation of its different parts, and by differences in the 

 quality of this light, that is, in its color. The former conditions are 

 fulfilled by an engraving, the latter by a painting. It is with the latter 

 conditions only, and with the power of i)erceiving them, that we are 

 concerned this evening. 



Belbre such an audience as that which 1 have the honor to adtlress, 

 it is unnecessary to say more about color than that it depends upon t!ie 

 power possessed by the objects which we describe as colored, to absorb 

 and retain certain portions of white or other mixed light, and torellect 

 or transmit other i)ortions. The resulting effe(!t of color is the imjues- 

 sion produced ui)on the eye or upon the brain by the waves of light 

 which are left, alter the process of selective absor[)lion has been accom- 

 plisheil. Some substances absorb two of the three funilamental colors 



