59 
FEesruary 27TH, 1854. 
THOMAS ROMNEY ROBINSON, D.D., Presivenr, 
in the Chair. 
Tue Secretary read a paper, by George James Knox, Esq., 
on the transmutation of metals. 
Mr. Thomas Grubb read the following account of a new 
method of determining, approximately, the spherical aberration 
of a combination of lenses for microscopic purposes :— 
«¢ The methods hitherto at our option for investigating the 
spherical aberrations of a system of lenses, having spherical 
surfaces, are—firstly, the purely mathematical, involving 
(where the thickness of the lenses is required to be included, 
and more especially where the angle of aperture is consider- 
able) such intricacy in the calculations as renders the process 
nearly useless to practical persons; and, secondly, the more 
practical method of constructing diagrams of large size, in 
which two or more rays, at different distances from the axis 
of the compound, are geometrically traced, according to the 
laws of optics. 
«‘ Much of my leisure time has, for several years, been 
devoted to inquiries including, necessarily, the construction 
of such diagrams ; and I cannot speak too highly of their value 
in giving to the experimenter in optics a thorough practical 
insight into the effects of the various forms and combinations 
which will be suggested, more especially when the improve- 
ment of the compound objective of the microscope is under 
consideration. 
‘“<Tt may be desirable here to state, that these diagrams 
were usually drawn on a scale ten times that intended for use, 
the radii of the arcs, from which the sines were measured 
