VISION. 225 



the distance of the second nodal point from the retina 

 usually 15 mm. d the diameter of the sketch of the 

 blind spot drawn on the paper, and D the correspond- 

 ing size of the blind spot: -p = ~|| or D = F j*. 



d. The macula lutea or yellow spot. Maxwell's experiment, 



(12) Make a strong solution of chrome alum filter it, 

 and place it in a clear glass bottle with flat sides.. 

 Close the eyes for a minute or so, open them, and 

 while holding the chrome alum solution between one 

 eye and a white cloud, look through the solution. An 

 oval or round rose spot will be seen in the otherwise 

 green field of vision. The pigment in the yellow spot 

 absorbs the blue-green rays, hence the remaining 

 rays which pass through the chrome alum give a rose 

 color. 



(13) Is it possible to calculate the size of the macula 

 lutea? 



e. Shadows of the fovea centralis and retinal blood vessels. 



Move, with a circular motion, a blackened card with 

 a pinhole in its center, in front of one eye looking 

 through the pinhole at a white cloud. Soon a punc- 

 tated field appears with the outlines of the capillaries 

 of the retina. The oval shape of the yellow spot is 

 also seen, and it will be noticed that the blood vessels 

 do not enter the fovea centralis. Move the card ver- 

 tically, when the horizontal vessels are most distinct. 

 On moving it horizontally, the vertical ones are most 

 distinct. Some observers recommend that a slip of 

 blue glass be held behind the hole in the opaque card, 

 but this is unnecessary. 



