ON PHYSIOLOGICAL LIMITS OF MICROSCOPIC VISIOIT. 471 



"We, however, habitually see through another yellow screen, 

 present throughout the whole extent of the retina, namely, the 

 narrow-meshed plexus of its capillary vessels which lies in front of 

 all the percipient elements. The quantity of the rays of the 

 spectrum, which a single layer of corpuscles, sometimes standing on 

 their edges and disposed like rouleaux of coin, absorbs, is very 

 considerable, as an examination with Browning's spectroscope 

 shows. The hoemoglobin lines are visible, and a considerable 

 portion of the rays at the violet end of the spectrum are lost. With 

 thicker layers of blood corpuscles, 'like those circulating in the 

 larger retinal vessels, the absorption effects would clearly be much 

 more considerable. Alterations in the blood affecting this absorption 

 power for certain luminous rays, must, necessarily, lead to unusual 

 perceptions of colour."^' 



And, here, another peculiarity of the retinal structure requires 

 notice. Nearly in the axis of vision, and at some distance to the 

 side of the optic nerve entrance, an intensely yellow pigment is 

 deposited between the elements of the different layers of the retina. 

 The centre of the ''yellow spot " is depressed on the surface 

 looking towards the front of the eye, to form the forea centralis. 

 This colouring matter, most intense in the fossa, is completely 

 hyaline, and only so far disturbs the transparency of the retina at 

 this part, that it absorbs a considerable portion of the violet and 

 blue rays before these reach the layer of cones. "With the aid of 

 Browning's spectroscope Max- Schultze has distinctly perceived the 

 shortening at the violet end of the spectrum under the microscope, f 



* An important fact must not be omitted, namely, that there are no retinal 

 blood vessels in front of the yellow spot. The significance of this fact is two-fold, 

 since there can be no entopic shadows thrown on this most sensitive part of 

 the retina by blood vessels or contained blood, whilst their absence at this 

 spot implies, that the deterioration of vision caused by such shadows really 

 does happen over those parts of the surface, outside the axis of vision, on 

 which images may be thrown. 



t The yellow spot is less sensitive to weak light than other parts of the 

 retina. It has been long known that many stars of inferior magnitude are 

 seen more brightly if looked at somewhat obliquely, than when their rays fall 

 full upon the eye. This can be proved to depend partly on the yellow color 

 of the yellow spot, which weakens blue more than other rays. It may also 

 be partly the result of the absence of vessels at this yellow spot, in con- 

 sequence of which the direct contact with life-giving blood fails. 



