1878.] determining the limits of microscopic vision. 



221 



For convenience the diameter of the webs were supposed to be 

 the jooo^ ^^ ^^ i^c^ i^^ diameter : those actually employed were the 

 Y^ogth and go-Vo*^ °f ^^ inch, both of which were seen in the 

 miniature cross wires; the calculated diameter of these under the 

 following object-glasses would therefore be 



Distance. 



Objective. 



Eecluction. 



Miniature diameters. 



6i 



1 i 



i immersion 



1875 



55 times less 



1 +1t 



7 000^*^ 



1 +1. 



3 8 5 0^^^ 



Beck 



Jg- immersion 



1878 



140 times less 



vth 



.th 



.th 



112000TJ 



til 



In round numbers the smallest spider-line appeared in the ^th 

 Beck miniature loooo oo^^ ^^ ^^ ^^^^ ^^ diameter, and it was viewed 

 with a magnificent Powell and Lealand, immersion i made ex- 

 pressly for me (price 11 guineas). Now if the microscope magnifies 

 one thousand times, it is interesting to inquire at what visual angle 

 the miniature of the smallest spider-line was seen as a diameter or 

 thickness diminished 140 times. 



arc 



Here^= -^=^^^ 

 rad. 



-r 140x 1000 

 10 



(at 10 inches) 



18|- seconds nearly. 



But I may here state it is by no means necessary to magnify 

 the lines 1000 times to ensure their visibility : 500 will shew 

 them, and then the visual angle is about nine seconds. I may 

 mention that the President of the Royal Microscopic Society, the 

 late Rev. J. B. Reade, F.R.S., informed me by letter he could 

 distinguish single telegraph wires J inch in diameter distant 

 800 yards, at a visual angle, that is, of less than two seconds 

 (1"'85). This acuteness of vision has been lately surpassed by 

 Mr Slack, P.R.M.S. 



He mounted cross hairs in a square frame upon a pole against 

 a grey sky and then against the background of a white wall, and 

 finally viewed them, together with some friends, and lastly ob- 

 served them with the sun glittering upon them. The distances 

 and visual anoles are as under. 



