Penetration into Space by Ttlefcopes, > 13 



This is not owing 4o the lead impcrfeftion in the mirror, which is truly parabolical, 

 and (hews, with all its aperture open, and a magnifying power of 600, the double ftar y 

 Leonis in the greateft perfeftion. 



" It fliewcd Venus, perfedly 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 neceffariiy 

 muft be fo, when carried to a high pitch, is evident ; for, by enlarging the aperture of the 

 telefcope, we increafe the evil that attends magnifying, which is, that we cannot magnify 

 the objeft without magnifying the medium. Now, fince the air is very feldom of fo 

 homogeneous a difpofition as to admit to be magnified highly, it follows that we muft 

 meet with impurities and obftru£lions, in proportion to its quantity. But the contents of 

 the columns of air through which we look at the heavens by telefcopes, being of equal 

 lengths, muft be as their bafes, that is, as the fquares of the apertures of the telefcopes ; 

 and this is in a much higher ratio than that of the increafe of the power of penetrating into 

 fpace. From my long experience in thefe matters, I am led to apprehend, that the higheft 

 power of magnifying may pofllbly not exceed the reach of a 20 or 25-feet telefcope; or 

 may even lie in a lefs compafs than either. However, in beautiful nights, when the out- 

 fide of our telefcopes is dropping with moifture difcharged from the atmofphere, there are 

 now and then favourable hours, in which it is hardly poflible to put a limit to magnifying 

 power. But fuch valuable opportunities are extremely fcarce; and, with large inftru- 

 ments, it will always be loft labour to obfervc at other times. 



As I have hinted at the natural limits of magnifying power, I fhall venture alfo to ex- 

 tend my furmifes to thofe of penetrating power. There feems to be room for a confide- 

 rable increafe in this branch of the telefcope ; and, as the penetrating power of ray 40-feet 

 refledor already goes to 191,69, there can hardly be any doubt but that it might be carried 

 to 500, and probably not much farther. The natural limit feems to be an equation be- 

 tween the fainteft ftar that can be made vifible, by any means, and the united brilliancy of 

 ftar-light. For, as the light of the heavens, in clear nights, is already vtry confiderable In 

 my large telefcope, it muft in the end be fo increafed, by enlarging the penetrating power, 

 as to become a balance to the light of all objeds that are fo remote as not to exceed in 

 brightnefs the general light of the heavens. Now, if P be put for penetrating power, we 



have ^/ ~ = A = 10 feet 5,2 inches for an aperture of a refledlor, on my conftruc- 



tion, that would have fuch a power of 500. 



But, to return to our fubjedl •,'"from what has been faid before, we may conclude, that 

 objefts are viewed in their greateft perfe£lion, when, in penetrating fpace, the magnify- 

 ing power is fo low as only to be fuflicient to ftiew the objedl well ; and when, in magni- 

 fying objefts, by way of examining them minutely, the fpace-penetrating power is no 

 higher than what will fuffice for the purpofe ; for, in the ufe of either power, the injudi- 

 cious overcharge of the other, will prove hurtful to perfect vifion. 



