80 Dr. Herschel on the Power of 



June 18, 1799. I viewed the planet Venus with a 10-feet 

 reflector. 



" Its light is so vivid that it does not require, nor will it bear, 

 " a penetrating power of 29, neither with a low nor with a 

 " high magnifying power." 



This is not owing to the least imperfection in the mirror, 

 which is truly parabolical, and shews, with all its aperture open, 

 and a magnifying power of 600, the double star y Leonis in 

 the greatest perfection. 



" It shewed Venus, perfectly well defined, with a penetrating 

 " power as low as 14, and a magnifying power of 400, or 600." 



Here, penetrating power was injurious to magnifying power; 

 and that it necessarily must be so, when carried to a high pitch, 

 is evident ; for, by enlarging the aperture of the telescope, we 

 increase the evil that attends magnifying, which is, that we 

 cannot magnify the object without magnifying the medium. 

 Now, since the air is very seldom of so homogeneous a dispo- 

 sition as to admit to be magnified highly, it follows that we 

 must meet with impurities and obstructions, in proportion to 

 its quantity. But the contents of the columns of air through 

 which we look at the heavens by telescopes, being of equal 

 lengths, must be as their bases, that is, as the squares of the 

 apertures of the telescopes ; and this is in a much higher ratio 

 than that of the increase of the power of penetrating into space. 

 From my long experience in these matters, I am led to appre- 

 hend, that the highest power of magnifying may possibly not 

 exceed the reach of a 20 or 25-feet telescope ; or may even lie in 

 a less compass than either. However, in beautiful nights, when 

 the outside of our telescopes is dropping with moisture dis- 

 charged from the atmosphere, there are now and then favourable 



