Angular AjJerture of Object-glasses. 117 



Fig. 4, brought iu to the centre, &, of the semi-cyHnder c, at which 

 there is a thin metal slit or stop of suitable diameter. 



Fto. 4. 



In this measurement I have rather over than under estimated 

 the aperture from using a stop too large ; less than sVth of an inch 

 would have been more proper. 



Believing that apertures, to be correctly measured in future, 

 must have all extraneous lateral rays cut off by a slit or stop of very 

 thin foil in the focus of the object-glass, and as this requires to be an 

 adjustable piece of apparatus, the following plan has been arranged. 



In order to prevent injury to the edges of the slit, and to keep 

 them in one plane, the two strips of foil (which may be of platinum) 

 are cut with a knife along a steel straight-edge. One of these pieces 

 is cemented on to a glass slip, the other is stretched across a per- 

 forated metal plate shding on the top of the glass, and adjustable by 

 a screw. The edges of the two strips abut against each other, and 

 are adjusted for parallelism by setting up the edge on the glass to 

 the other while the cement is warm. The instrument resembles 

 an eye-piece micrometer. The glass surface serves to focus upon, 

 by which the coincidence with the anterior plane of the slit is 

 ensured. The thickness of glass beneath will not alter the angle of 

 aperture, as this is the same both for incidence and emergence. 

 The arrangement is placed on the microscope stage, which should 

 be a very thin one ; the slit is adjusted to the proper width, and 

 brought centrally in the field, and the object-glass focussed on to the 

 glass surface, with the microscope in a horizontal position, and the 

 lamp flame a foot or two away. I have an instrument rotating 

 from a centre pin in its own base, which is divided ; but in lieu of 

 this it may be set on a small wooden turn-table divided at its edge. 



