68 Dr. Herschel on the Power of 



From the result of this computation it appears, that the cir- 

 cumstance of seeing so well, in the dusk of the evening, may- 

 be easily accounted for, by a power of this telescope to penetrate 

 39 times farther into space than the natural eye could reach, 

 with objects so faintly illuminated. 



This observation completely refutes an objection to telescopic 

 vision, that may be drawn from what has also been demon- 

 strated by optical writers ; namely, that no telescope can shew 

 an object brighter than it is to the naked eye. For, in order to 

 reconcile this optical theory with experience, I have only to say, 

 that the objection is intirely founded on the same ambiguity 

 of the word brightness that has before been detected. It is 

 perfectly true, that the intrinsic illumination of the picture on 

 the retina, which is made by a telescope, cannot exceed that of 

 natural vision; but the absolute brightness of the magnified 

 picture by which telescopic vision is performed, must exceed 

 that of the picture in natural vision, in the same ratio in which 

 the area of the magnified picture exceeds that of the natural 

 one ; supposing the intrinsic brightness of both pictures to be 

 the same. In our present instance, the steeple and clock-dial 

 were rendered visible by the increased absolute brightness of 

 the object, which in natural vision was 15 hundred times inferior 

 to what it was in the telescope. And this establishes beyond a 

 doubt, that telescopic vision is performed by the absolute bright- 

 ness of objects ; for, in the present case, I find by computation, 

 that the intrinsic brightness, so far from being equal in the tele- 

 scope to that of natural vision, was inferior to it in the ratio of 

 three to seven. 



The distinction between magnifying power, and a power of 

 penetrating into space, could not but be felt long ago, though 



