372 THE SENSORIAL FUNCTIONS. 



nary sensibility, for the purpose of observing very faint ce- 

 lestial objects. It often happened to him, when, in a fine 

 winter's night, and in the absence of the moon, he was oc- 

 cupied during four, five, or six hours in taking sweeps of 

 the heavens with his telescope, that, by excluding from the 

 eye the light of surrounding objects, by means of a black 

 hood, the sensibility of the retina was so much increased, 

 that when a star of the third magnitude approached the 

 field of view, he found it necessary immediately to with- 

 draw his eye, in order to preserve its powers. He relates 

 that on one occasion the appearance of Sirius announced 

 itself in the field of the telescope like the dawn of the morn- 

 ing, 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 impressions made on parti- 

 cular parts of the retina from those made on the rest, and 

 from their general effect on the whole surface. These par- 

 tial variations of sensibility in the retina give rise to the phe- 

 nomena of ocular spectra, , as they are called, which were 

 first noticed by Buffbn, and afterwards more fully investi- 

 gated by Dr. Robert Darwin. A white object on a dark 

 ground, after being viewed steadfastly till the eye has be- 

 come fatigued, produces, when the eye is immediately di- 

 rected to another field of view, a spectrum of a darker co- 

 lour 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 re- 

 tina 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 



