OCULAR SPECTRA. 471 



found it necessary immediately to withdraw his 

 eye, in order to preserve its powers. He relates 

 that on one occasion the appearance of Sirius an- 

 nounced itself in the field of the telescope, like the 

 dawn of the morning ; increasing by degrees in 

 brightness, till the star at last presented itself with 

 all the splendour of the rising sun ; obliging him 

 quickly to retreat from the beautiful but over- 

 powering spectacle. 



The peculiar construction of the organ of vision 

 allows of our distinguishing the effects of impres- 

 sions made on particular parts of the retina from 

 those made on the rest, and from their general 

 effect on the whole surface. These partial variations 

 of sensibility in the retina give rise to the phe- 

 nomena of oculat' spectra, as they are called, which 

 were first noticed by BufFon, and afterwards more 

 fully investigated by Dr. Robert Darwin. A white 

 object on a dark ground, after being viewed stead- 

 fastly till the eye has become fatigued, produces, 

 when the eye is immediately directed to another 

 field of view, a spectrum of a darker colour than 

 the surrounding space, in consequence of the ex- 

 haustion of that portion of the retina on which its 

 image had been impressed. The converse takes 

 place, when the eye, after having been steadfastly 

 directed to a black object on a light ground, is 

 transferred to another part of the same field, and in 

 this case a bright spectrum of the object is seen. 



It is a still more curious fact that the sensibility 

 of the retina to any particular kind of light, may, 

 in like manner, be increased or diminished, without 

 any change taking place in its sensibility to other 

 kinds of light. Hence the spectrum of a red object 



