ry 
MEMOIRS. 
ReEsEARCHES on the Errors of MicroscopicaL Vision, 
and on NEw Metuops of CorrEcTING THEM. By G. W. 
Royston-Picortr, M.A., M.D. Cantab., M.R.C.P., Fel- 
low of the Cambridge Philosophical Society, of the Royal 
Astr. and Mic. Soc. of London; formerly Fell. St. Pet. 
Coll., Cambridge. 
(Part I, with Plate I.) 
Ir would be unjust to the memory of Mr. Lister to omit 
the statement here that microscopical science is more deeply 
indebted to his discovery of “ two aplanatic foci” than to any 
ether, except the principles of achromatism. Published ori- 
ginally by the Royal Society in their ‘ Transactions,’ this 
discovery of practical APLANATIC FOCI acquired a world-wide 
celebrity, and in a comparatively short space of time the 
microscope took gigantic strides from the power represented 
by 200 linear to one fivefold greater, with a corresponding 
increase of brilliancy and sharpness of definition. The aper- 
ture, raised from 30 degrees to 170, conferred new powers of 
resolution. Objectives were gradually constructed from one 
quarter to the fiftieth of an inch focal length. Proposed by 
him also was a method of correcting the aberration intro- 
duced in a well corrected objective by covering the object 
with a thin lamina of glass. The powers of the best con- 
structed microscope depend, unlike the telescope, upon the 
variable distance between the eye and object-glass, and are 
a function of this distance and the product of the Herschelian 
powers of the eye-piece and objective.’ 
1 Tt may be convenient here to repeat that, 10 inches being taken as the 
standard distance of distinct vision, ~ is the power of an eye-piece of focal 
length E, ” that of an objective of focal length F, so that the magnifying 
power at a distance of 10 inches is 
TO 1044. 100 
= E*% 5 = oF 
VOL. XI.—NEW SER. A 
