Limits of Microsc(ypio Vision. By Br. Boyston-Pigott. 15 



to put a double set of cross wires, and also a set of parallel wires, 

 in the micrometer. The head of the instrument is divided into one 

 hundred parts, and a half or quarter part is readily seen with the 

 naked eye. I may here observe, when the wires are reduced 

 thirty-eight times by the |^ (as one division of the micrometer is 

 the roAo of ail i^^ch of motion in the wires), a single division for 

 the miniature then reckons ^^o"(Toth. But I found a quarter of a 

 division made a perceptible difference in the apparent thickness of 

 two coincident webs ; whilst three ivhole divisions separated the 

 webs so completely., that a narrow strip of light could be discerned 

 between them (not much room here for swellmg or enlargement of 

 the lines !). 



I then changed the glasses, putting the best glass in the body, 

 and the older one (both newly formulated) in the micrometer : the 

 definition was not so good. It required 3^ divisions to separate the 

 same lines. 



This dividing of close lines by means of a very finely con- 

 structed micrometer is quite satisfactory to my mind, and I should 

 hope conclusive as a crucial test to others who may witness it that 

 the lines are very truly portrayed. 



The following little circumstance has an interest of its own. 

 Having conveyed my instruments home from the London Museum, 

 S.K., I found the webs entirely covered with London dust. Upon 

 getting them, however, into rapid vibration, I succeeded in shaking 

 off nearly the whole before measuring them. A few minute particles 

 adhere here and there ; and though these webs are diminished fifty 

 times — i.e. to the 300, 000th of an inch — these particles .of dust 

 are visible on the web in this state of reduction. This result is the 

 most surprising of all. 



It was found that under this reduction (fifty times) it required 

 five divisions to separate the spider lines, or a movement of jo o^^ 

 of the micrometer, i. e. 



5 _ 5 _ 1 

 50 X 10000 ~ 500000 ~ 100000 " 



Each division represented here on the micrometer head 



1 



500000 



of an inch 



in the field of view of the Microscope. 



It is interesting to inquire what effect separating the spider 

 lines has upon the discriminating power of vision. The optical 

 conditions of seeing a black line upon a white ground, and sepa- 

 rating or clearly dividing between two close minute black lines, are 

 totally different. The researches of Dr. Jurin, 150 years ago, and 

 of Dr. Eobinson, F.E.S., the astronomer, on the subject, are very 

 interesting ; but no observations have yet been made of the minute- 



