^L 



Transactions of the Society. 



whole subject to the public through the medium of the Eoyal 

 Society. 



" Should this be the case, I beg your acceptance of the enclosed ; 

 but if you have no objection to Mr. Delarue's proposal, I will thank 

 you to return it to me, and I will send you as many of the printed 

 cards as you may wish to have. I would on no account allow the 

 printing to take place without your permission. 



" Can you inform me what is the smallest space visible to the 

 unassisted eye, either in parts of an inch, at a certain distance, or 

 in parts of a circle ? I want to calculate the magnifying power 

 which it will be necessary to apply in order to obtain the full 

 amount of separation which an object-glass of a certain aperture is 

 capable of effecting. 



I am, 



Dear Sir, 



Yours very truly, 



Geoege Jackson. 



J. J. LiSTEB, Esq." 



[Enclosure.J 



A table of the smallest separation visible by ?nea?is of the Microscope^ 

 between the centres of parallel lines or squares^ iviih object-glasses of 

 different angles of aperture : calculated by a formida deduced from 

 Mr. Lister's observations. 



BTrrr 



5T2T5 

 5¥3¥2 



TTJ2TT 

 T2¥^ff 



5(7523 

 52^7^ 

 5J¥TT 

 5B3'T5 

 5T^5^ 



[It will be found by trial that the formula used to calculate these 

 •"separations" was 0*0000105 in. divided by the sine of half the angle 

 of aperture ; or turned into the form more familiar at the present day, 

 it was : 



Resolution equal to 95,240 hnes per inch multiplied by the 

 numerical aperture of the objective, almost exactly the formula which 

 is in favour now on the basis of Abbe's work. 



