Microscopical Researches in High Power Definition. 265 



mustard seed, separated by one-third the diameter of either. VicincE 

 according to Struve, but at least per vicince : dist. between centres 

 2-3rds of a sec."* 



Recurring again to the law of brilliant disks in the microscope dimi- 

 nishing in diameter with the increase of objective angular aperture, 

 I beg to call attention to the observation that in examining minia- 

 tures reduced 1,000 times, the details of these miniatures appeared 

 entirely free from diffraction effects so long as the light was subdued. 

 But the instant the sun shone every reflecting point or polished surface 

 became entirely disguised, concealed, or obliterated in its details by 

 the strong diffractions. f It is on this account that mild daylight 

 observations from light received from a white cloud have been found 

 so serviceable in minute research as already described. 



Unfortunately, the attainment of the most delicate microscopic 



* From these data, if 6x be diameter of each, 2a? is the dividing space or " black 

 division," so that 



3x + 2x + 3x = %". 

 Whence 6x or diameter of disk = \'\ 



2x or division = t?"- 

 Hence the angle subtended by either disk with power 320, 



= 320 x £" = 160". 

 The angle subtended by dividing black line, 



e = 320xi" = 53". 



The last result corresponds to a black line l-400,000th of an inch magnified 1,000 

 times in the field of the microscope. 



A short table is here inserted for reference. If the millionth of an inch be taken 

 as the microscopic unit of reference (m) , and a particle or line of this size be mag- 

 nified 1,000 times, it will subtend to the eye of the observer, as an image 10 inches 

 distant, an angle ten times greater than the human limit, viz. : — 



20"-6265. 



Diameter of object. 



Visual angle. 



Power. 



l-10,000,000th 



"2 



1,000 



1- 1,000,000th 



20k 



1- 500,000th 



41 





1- 400,000th 



5H 





1- 300,000th 



69 





1- 200,000th 



103 





1- 100,000th 



206 





1- 50,000th 



412* 





l-l,000,000th 



l-l,000,000th 



41 

 60 



2,000 

 2,500 



f See " Circular Solar Spectra," by the writer, " Proc. Roy. Soc," vol. 21, p. 426 



