1 8 1 Transactions of the Society. 



period of thirty-five years, has described it in this Journal, 1899, 

 p. 209, where the measurements of its various parts will be found ; 

 but the Varley foot and the conical joint are described in the 

 Journal, 1900, p. 291. 



Since the reading of this paper, Mr. Parsons has found a notice 

 (d.ite 1854) of the firm of opticians, Watkina and Hill, at Charing 

 Cross. 



Erratum. — A correction is needed in my note on Watkins 

 Microscope (this Journal, 1908, p. 141, line 26). For "... an 

 objective which was a combination," read " .... a dividing ob- 

 jective." A dividing objective must obviously be a " combina- 

 tion," but the reverse is not necessarily true. The word "combina- 

 tion " in technical optics has a special meaning, and is only used 

 to denote an objective whose lenses, or groups of achromatic lenses, 

 are separated from one another. For example, an object-glass con- 

 sisting of two or more achromatic doublets is a " combination," but 

 a single achromatic doublet would be called a " doublet." Similarly 

 the historically famous triple inch by Tulley would be called a 

 " triple," and not a " combination." 



The first Microscope to possess a " combination " was that of 

 Grindl, described in Zahn's Oculus Artificialis, 1G85. 



