366 STUDIES IN SPECIAL SENSE PHYSIOLOGY 



With other colours, except red, the same result is obtained, but 

 less distinctly. 



Although anticipating our theoretical summary, I must point out 

 how strongly this suggests the activity of two distinct mechanisms 

 with different latent periods. An analogous, but not identical, 

 result is the old illusion of the " fluttering hearts." Small heart- 

 shaped scraps of blue paper are pasted upon a red surface and the 

 whole examined in dim light. On moving the sheet backwards 

 and forwards, the blue scraps appear to lag behind the red back- 

 ground. This phenomenon does not occur on stimulation of the 

 fovea centralis. 



The secondary, ghost, or satellite, begins \- to 1 second after the 

 commencement of the primary, and is, in general, complementary 

 to it. This rule must, however, be modified in the following way. 

 If the primary be pure white, the secondary is bluish ; indeed, the 

 secondary is always modified in the direction of bluishness. Even 

 with a feeble blue primary, the secondary may still be bluish. It 

 is only with a saturated blue primary that a secondary of a really 

 complementary (yellow) hue is obtained. As regards brightness, 

 the result depends on the adaptation value of the primary. Two 

 lights of equal stimulus values for the " dark " eye give equally 

 bright secondaries. Ked, with its relatively low stimulating power 

 as regards the " dark " eye, only gives a secondary when its physical 

 brightness is great. 



As we should expect from its characteristics, the secondary is 

 dependent on dark adaptation, increasing to a maximum of dis- 

 tinctness as adaptation proceeds and then diminishing, although 

 Macdougall has obtained it after prolonged dark adaptation. We 

 thus see in the causative factors of the secondary image the pheno- 

 mena we have already learned to associate with peripheral vision : 

 (1) Relatively greater efficiency of short waves ; (2) increased in- 

 tensity with increasing dark adaptation. It only remains to add 

 failure over the fovea centralis. " If one observes it in the form 

 of an after-coming image and fixates carefully a luminous point in 

 the track of the object, one sees clearly that the satellite leaps 

 over a small central area, while it follows without a break similar 

 points of light moved over a paracentral region. Also, with 

 stationary objects, momentarily illuminated, of suitable form and 

 size, an analogous appearance can be demonstrated. Small objects 

 which lie entirely within the foveal region do not exhibit the 



