20 Transactions of the Society. 



angle of 20 SGconds, and probably a good deal less, as 2 seconds 

 is the visual limit that can be seen in the apparatus or by the eye 

 alone. 



Another very curious point is worth mentioning. Dr. Jurin 

 150 years ago stuck two pins on a window-pane, and found that 

 v/hen placed near each other he could not divide them except when 

 the interval between them reached the wide visual angle of 30". 

 But when only one pin was viewed, he could distinguish it at a 

 visual angle of from ^ to 3 seconds ! 



This interesting fact explains what I have witnessed in sepa- 

 rating the spider lines of the micrometer in these miniatures : the 

 interval could only be seen when the lines were separated, centre to 

 centre, three divisions (micrometer), each division rejjresenting 

 -^^^ — when a minuendo oi fifty times was employed ; yet one can 

 see a most sensible thickening of the gossamers just beginning to 

 separate by moving the micrometer half a division. From this, I 

 presume, a similar phenomenon was produced, though very much 

 less pronounced than Jurin's case. It is marvellous to me that a 

 visible bright space between these lines can be seen at all when 

 their centres are sejiarated only three divisions, i. e. ^ ^ ^ 'j^ ^ ^ ^ or 

 — ^ — of an inch. Considering that there must be some resi- 



3 . ^ 



duary aberration, however small, and that the error of each set of 

 glasses accumulates in the final image presented to the eye, it seems 

 to me wonderful that, notwithstanding Jurin's fact, a division is 

 visible between the gossamers at all with so light a movement as 

 described. 



In continuation of this subject, I propose to offer to the Society 

 some researches on the efiect of large and small apertures in object- 

 glasses. I beg to commend this research to the earnest attention 

 of the rising generation of microscopists. Unless I am very much 

 mistaken, the idea propagated in reference to the limits of micro- 

 scopic vision is totally erroneous ; whilst for brilliant lines or 

 minute disks of great brilliance, I have not the slightest hesita- 

 tion in embracing the truths conveyed in the exquisite formula 

 presented to the microscopical world by, I believe, independently, 

 Professors Helmholtz and Abbe. 



It is almost needless to remark that very firm supports and 

 delicacy of the adjustments as regards spherical aberration and 

 illumination are essential to the success of this refined kind of 

 definition. 



