18 Transactions of the Society. 



These astonisliing results, so contrary to what had been gene- 

 rally supposed, demand thorough investigation. And with a view 

 to elucidate this unusually important subject, it will be interesting 

 to inquire what is the visual angle of fine-line objects just visible 

 by different observers. 



It is now requisite to determine what would be the visual angle 

 of the spider line iroVo of an inch miniatured 140 times smaller 

 with the Beck 4o, and then magnified up 1000 times by an eighth 

 immersion with C eye-piece and about 10 inches of tube. Here 



Visual diameter of web S = — -— -¥ 140 x 1000 at a distance of 10 iuches. 



oOOu 



Hence 



Pern. 5 1000 1 



''''■ ' = Rl^dhTs == To = iTu-iaoooiao = moo = ^' ''''""^' ^^"■^^- 



The most ready w^ay of getting the value of the fraction in 

 seconds is by recollecting that 60" = ^4^^ nearly. 



deferring now to the former table, it will be found by simple 

 arithmetic that since the Beck oV immersion shows theoretically a 

 visual angle of 18 seconds, miniaturing 140 times, a glass reducing 

 only fifty-eight times ought to show at an angle of 7 J seconds at a 

 power of 1000, and at a power of 500 at about 4 seconds. I see 

 the line plainly, most charmingly defined with 500, and can even 

 see them when miniatured only thirty times. A good deal might be 

 written on this extraordinary fact. As the aperture of the objec- 

 tives is diminished the spider lines look blacker, and therefore 

 larger,' I reserve this question for future treatment. 



In inferior glasses the spider lines are thickened, and, besides 

 this, garnished with secondary lines, true diffraction lines, and this 

 you may see. I first detailed the metliod of miniatures in the 

 ' Philosophical Transactions ' eight years ago ; but I have had 

 nearly twelve years' experience of this method, and I have several 

 times recommended it to the microscopical world with great cor- 

 * The glittering line here would afford a broad spurious line greatly enlarged. 



