1878.] Dr Royston-Figott, On a new metJiod, etc. 217 



December 2, 1878. 

 Professok G. D. Liveing, President, in the Chair. 



The following communications were made to the Society : — 



(1) Dr G. W. RoYSTON-PiGOTT, M.A., M.D, F.RS., On a neiu 

 method of determining the limits of microscojnc vision. 



The method I am about to describe, that of forming minia- 

 tures, has already been published in the Philosophical Transactions^ 

 and Proceedings, but a novel application leads to some interesting 

 results which may not be unworthy of the attention of the 

 Society. 



I earlj became aware that a miniature of a given object, formed 

 by reversing an object-glass, was the severest possible test of the 

 excellence of the glass when examined by another object-glass of 

 high excellence ; viz. one in which the rays both central and peri- 

 pheral were both achromatic and aplanatic, and also in which the 

 mechanical working and centreing were of the highest attainable 

 perfection. These studies were rewarded with some remarkable 

 results, described in a paper on "Circular Solar Spectra" {Proc. 

 Roy. Sac. No. 146). 



A new application of the method is to form miniatures gradu- 

 ually reduced ten, twenty, fifty, and even a hundred and fifty 

 times, by a reversed objective placed centrally beneath the stage 

 of a microscope, and fixed firmly, yet completely under command, 

 by focussing and rectangular movements of the substage. The 

 miniature is then cautiously brought into the field of view of 

 the observing midroscope. See Fig. 1, Plate IV. 



By preference I have adopted fine spider lines, which are 

 remarkably true and apparently cylindrical : the liquid gum 

 naturally taking that form on being subjected to contraction and 

 tension. Those I have used most are about the ten-thousandth 

 of an inch in diameter as measured with a delicate micrometer. 

 Dr Brewster measured some as small as -g^ig^th. 



There is a good deal of adjustment required to keep the minute 

 field of view presented by the miniature exactly in focus and 

 central. Indifferent glasses, i.e. glasses of inferior quality, at once 

 present a perspective obscured by white fog, which denotes spheri- 

 cal aberration, or colour which demonstrates 'failure in achro- 

 matism. 



Now high class objectives of half an inch focal length and 

 downwards are furnished with a screw collar for regulating the 



1 Vol. II. 1870. 



16—2 



