TELESCOPES. 93 



they lose in intensity of light by dilatation in the magnifying 

 telescope. It must be further observed, that the apparent 

 motion of the fixed star, as well as of the planetary disc, is 

 increased by high magnifying powers. This circumstance may 



planet's image may be calculated by the proportions we have 

 already given. The total quantity of light contributing to 

 form the whole of the image, as seen by the naked eye, will, 

 therefore, be to the total quantity of the light forming the 

 image of the planet by the aid of a telescope, as the surface 

 of the pupil is to the surface of the object-glass. The com- 

 parative intensities, not of mere isolated points, but of the 

 images of a planet formed respectively on the retina of the 

 naked eye, and by the intervention of a telescope, must evidently 

 diminish proportionally to the superficial extent of these two 

 images. The linear dimensions of the two images are to one 

 another as the diameter of the object-glass is to that of the 

 emerging pencil. We therefore obtain the number of times 

 that the surface of the magnified image exceeds the surface 

 of the image when seen by the naked eye by dividing the 

 square of the diameter of the object-glass by the square of the 

 diameter of the emerging pencil, or rather the surface of the 

 object-glass by the surface of the circular base of the emerging 

 pencil. 



*" By dividing the surface of the object-glass by the surface 

 of the pupil, we have already obtained the ratio of the total 

 quantities of light produced by the two images of a planet. 

 This number is lower than the quotient which we obtain by 

 dividing the surface of the object-glass by the surface of the 

 emerging pencil. It follows, therefore, with respect to 

 planets, that a telescope causes us to gain less in intensity of 

 light than is lost by magnifying the surface of the images on 

 the retina; the intensity of these images must therefore 

 become continually fainter, in proportion as the magnifying 

 power of the telescope increases. 



"The atmosphere may be considered as a planet of indefinite 

 dimensions. The portion of it that we see in a telescope 

 will therefore also be subject to the same law of diminution 

 that w r e have indicated. The relation between the intensity of 

 the light of a planet and the field of atmospheric light through 



