BRIGHTNESS OF THE IMAGE. 



on the planet Saturn, and its visible area would be a hundred 

 times less than it is, but the splendour of its diminished area 

 would be exactly the same as the present splendour of the sun's 

 disk. 



The sun seen from the planet Saturn has an apparent diameter 

 ten times less than it has when seen from the earth. 



The appearance from Saturn will then be the same as would be 

 the appearance of a portion of the disk of the sun seen from the 

 ^arth through a circular aperture in an opaque plate, which would 

 exhibit a portion of the disk whose diameter is one-tenth of the 

 whole. 



45. "When the light which radiates from a luminous object has 

 a certain intensity, it will continue to affect the retina in a 

 sensible manner, even when the object is removed to such a dis- 

 tance that the visual angle shall cease to have any perceivable 

 magnitude. The fixed stars present innumerable examples of 

 this effect. None of these objects, even the most brilliant of them, 

 subtend any sensible angle to the eye. When viewed through 

 the most perfect telescopes they appear merely as brilliant points. 

 In this case, therefore, the eye is affected by the light alone, and 

 not by the magnitude of the object seen. 



46. Nevertheless the distance of such an object may be increased 

 to such an extent that the light, intense as it is, will cease to pro- 

 duce a sensible effect upon the retina. 



There are seven classes of the fixed stars, diminishing gradually 

 in brightness,* which produce an effect on the retina such as to 

 render them visible to a naked eye. This diminution of splendour 

 is produced by their increased distance. The telescope brings into 

 view innumerable other stars, whose intrinsic splendour is as 

 great as the brightest among those which we see, but which do not 

 transmit to the retina, without the aid of the telescope, enough of 

 light to produce any sensible effect. It is demonstrable, however, 

 that, even without the telescope, they do transmit a certain 

 definite quantity of light to the retina ; the quantity of light 

 which they thus transmit, and which is insufficient to produce a 

 sensible effect, having to the quantity obtained by the telescope 

 a ratio depending upon the proportion of the magnitude of the 

 object-glass of the telescope to the magnitude of the pupil. 



47. The quantity and intensity of the light transmitted by an 

 external object to the retina, which is sufficient to produce a per- 

 ception of such object, depends also upon the light received at the 



* The term magnitude is used in astronomy, as applied to the fixed 

 stars, to express their apparent brightness ; no fixed star, howerer splendid, 

 subtends any sensible angle. 



p 2 67 



