1873.] On a new Formula for a Microscope Object-glass. Ill 



+ 



, * \ 



H 3 and 3NH 3 N and 3N0 3 HNH 3 ( = 3A T 3 + 4NH 3 ). 



Positive electrodes of silver, lead, copper, zinc, and magnesium are 

 dissolved by the liquid as (aninioniated) nitrates. A positive electrode of 

 mercury is converted into a compound almost insoluble in the liquid. 

 When the electrode is acted upon, the generation of nitrogen does not 

 take place. 



January 16, 1873. 



T. ARCHER HIRST, Ph.D., Vice-President, in the Chair. 



Pursuant to notice given at the last Meeting, Mr. Spottiswoode pro- 

 posed and Admiral Richards seconded the Right Hon. Hugh Culling 

 Eardley Guilders, Chancellor of the Duchy of Lancaster, for election and 

 immediate ballot. 



The ballot having been taken, Mr. Guilders was declared duly elected. 



The following communications were read : — 



I. " A new Formula for a Microscope Object-glass." By F. H. 

 Wenham. Communicated by W. B. Carpenter, M.D., 

 F.R.S. Received October 31, 1872. 



A pencil of rays exceeding an angle of 40° from a luminous point can- 

 not be secured with less than three superposed lenses of increasing focus 

 and diameter, by the use of which combination rays beyond this angle are 

 transmitted, with successive refractions in their course, towards the posterior 

 conjugate focus : until quite recently, each of these separate lenses has been 

 partly achromatized by its own concave lens of flint glass, the surfaces in 

 contact with the crown glass being of the same radius, united with Canada 

 balsam ; the front lens has been made a triple, the middle a double, aud 

 the back again a triple achromatic. This combination therefore consists 

 of eight lenses, and the rays in their passage are subject to errors arising 

 from sixteen surfaces of glass. 



In the new form there are but ten surfaces, and only one concave lens 

 of dense flint is employed for correcting four convex lenses of crown glass : 

 as this might at first sight be considered inconsistent with theory, a brief 

 retrospect of the early improvements of the microscope object-glass 

 will help to define the conditions. The knowledge of its construction 

 has been entirely in the hands of working opticians ; and the information 

 published on the subject being scanty, this has probably prevented the 

 scientific analyst from giving that aid which might have been expected. 



Previous to the year 1829 a few microscopic object-glasses were made, 

 composed of three superposed achromatic lenses ; but this combination 



