292 Dr Brewster on some Affections of the Retina. 



to faint impressions than the central ones, which may serve to 

 account for this phenomenon." * 



As it is with much diffidence that I venture to controvert 

 any opinion entertained by Mr Herschel, I have been at some 

 pains to investigate the subject experimentally. I was, at first, 

 disposed to ascribe the evanescence of the faint star, solely to 

 the same cause as the evanescence of faintly illuminated sur- 

 faces, and the reappearance of the star by indirect vision, to 

 the circumstance of the retina recovering its tone, by contem- 

 plating another object sufficiently luminous for vision ; but 

 this opinion was not well founded. 



If a given quantity of light, which is unable to afford 

 a sustained impression when expanded over a surface, is con- 

 centrated into a luminous point, it is still less fitted for 

 the purposes of vision. It then acts upon the retina some- 

 what in the same way as a sharp point does upon the skin. 

 The luminous point will alternately vanish and reappear ; and 

 if the retina is under the influence of a number of such points, 

 it will be thrown into a state of painful agitation. The same 

 effect is produced by a sharp line of light ; the retina is, in 

 this case, thrown into a state of undulation, so as to produce 

 an infinite number of images parallel to the luminous line ; 

 and when this line is a narrow aperture held near the eye, a 

 sheet of paper, to which it is directed, will appear covered 

 with an infinity of broken serpentine lines parallel to the aper- 

 ture. When the eye is stedfastly fixed, for some time, up- 

 on the parallel fines which are generally used to represent the 

 sea in maps, the lines will all break into portions of serpen- 

 tine lines, and red, yellow, green, and bine tints will appear in 

 the interstices of them. 



The evanescence of stars, therefore, of the last degree of 

 faintness, must be ascribed, both to their deleterious action 

 upon the retina as points of fight, and to the insufficiency of 

 their light to maintain a continued impression upon the retina. 



• If we recollect rightly, a similar fact, with regard to the satellites of 

 Saturn, is recorded in a late number of the Ann. de Chimie, and a similar 

 explanation given. It was, we think, noticed by some of the astronomers 

 in the Royal Observatory of Paris; but we bs^e not the Number at hand 

 to refer to. 



