282 



The announcements of meetings for the ensuing month 

 were then made, and the meeting terminated with the usual 

 conversazione. 



December 20th, 1901. — Ordinary Meeting. 



The PvT. Hon. Sir Ford North, F.R.S., Yice-President, 



in the Chair. 



Th-e minutes of the meeting of November 15th, 1901, were 

 read and confirmed. 



The following gentlemen were balloted for and duly elected 

 members of the Club : — Mr. Thomas Craig, F.R.M.S., Mr. 

 Frederick H. Hicks, Mr. Howard E. Hurrell, Lieut-Col. Thomas 

 Lamb. 



The donations to the Library were announced, and the thanks 

 of the Club unanimously voted to the donors. 



Mr. Angus being called upon to explain some apertometers 

 which he exhibited in the room, said that perhaps the aperto- 

 meter best known to most persons present, at least by name, 

 was that of Professor Abbe. Few amateurs were, however, 

 acquainted with it, as its application was limited, and it was 

 expensive. He had therefore brought with him and placed upon 

 the table some simpler forms, which could be made for very little 

 indeed. The first was merely a protractor, mounted on edge with 

 a piece of silvered cover-glass, in which, with a low-power object- 

 glass, a good view could be obtained of the protractor. Two 

 points marked off would indicate the angle, but they could also 

 get this by using a small pencil of light. This very simple con- 

 trivance of course showed the angle of the objective, and not the 

 numerical aperture. Another method shown was by placing a 

 disc of glass, with a millimetre scale ruled upon it, in the carrier 

 of an Abbe condenser. On putting this into position, by proper 

 adjustment of the draw-tube, and with an objective from | in. 

 to 2 in,, a very good view could be obtained of the millimetre 

 scale. To determine the value of this it was, hcAvever, necessary 

 to get an objective of known aperture for comparison. This 

 contrivance was also limited in its application, but he wanted to 



