4 8 4 SCIENCE PROGRESS 



again a bluish violet circle appears at the periphery or middle 

 of the field of vision, contracts, and then, after breaking up into 

 a star-shaped figure and becoming brighter, disappears to be 

 followed by another contracting circle. If the eye be opened 

 when the star figure has formed in the centre, it will appear as 

 a bright rose-coloured star, much brighter than any other part 

 of the field of vision. If, however, we wait till the star has 

 broken up and disappeared before opening the eye, it will be 

 found that only a black spot is seen in the centre. 



This is explained on the theory that when there is visual 

 purple in the fovea this is the most sensitive portion of the 

 retina ; when there is none there, it is blind. It also shows 

 conclusively that the fovea is sensitised from the periphery. 



Disappearance of Lights falling upon the Fovea. — If we look at 

 two small isolated stars of equal magnitude, either may be made 

 to disappear by looking fixedly at it, whilst the other remains 

 conspicuously visible. The phenomenon is most marked on a 

 dark night, and when the star looked at is in a portion of the 

 sky comparatively free from other stars, and when one eye is 

 used. On a very dark night a considerable number of small 

 stars, occupying the centre of the field of vision, may be made to 

 disappear, whilst stars occupying other areas of the field of 

 vision are plainly visible. This fact shows that when the visual 

 purple in the fovea is used up and not renewed, the latter 

 is blind. 



Currents seen in the Field of Vision not due to the Circulation (37). 

 — There are numerous methods by which currents in the field 

 of vision which are not due to the circulation can be seen. The 

 following is one example : 



If one eye be partially covered with an opaque disk whilst 

 both eyes are directed forwards in a not too brightly illuminated 

 room, and special attention be paid to the covered eye, an 

 appearance of whirling currents will be seen with this eye. 

 These currents appear to be directed towards the centre and 

 have a very similar appearance to a whirlpool. On closing both 

 eyes all the portion in which the whirling currents are seen 

 appears as dull purple. These currents cannot be due to vessels, 

 because we know that the centre of the retina corresponding to 

 the point where the greatest movement is seen, is free from 

 vessels. The appearance is also very different from that of the 

 movement of blood in vessels. The experiment succeeds best if 



