200 Dr Searle, The determination of the effective 
The diameter d of the hole in the plate may be measured by a) 
glass scale placed with its divided face in contact with the plate. | 
If desired, greater accuracy could be obtained by using a travelling 
microscope. 
$6. Third method. The determination of the distance BQ, 
may be avoided by using two plates with apertures of diameters 
d, and d,. Let ¢, and c, be the diameters of the corresponding 
bright patches on the matt surface of the glass scale. Since, by 
(5), € — @ 1s proportional to d, we have ) 
: (c, — a)/(c, — a) = d,/ds. 
Thus a (d, — d.) = c,d, — ¢,d, = c, (d, — dz) — d, (4, — &), 
d(C; — C2) 
1 eg ee er (6) 
and hence vt 
§7. A system for laboratory work. A projection lens system 
such as is used in optical lanterns is convenient for the experiment. 
Such lenses are moderate in price (10/- upwards) and are reason- 
ably well corrected. The cylindrical tube which forms the mount 
of the lens is, on account of its form, easily attached to a base. 
The stop may be inserted in the tube between the two components 
of the lens system. Fig. 4 shows the arrangement which has been 
found convenient at the Cavendish Laboratory. The projection 
lens AB, of about 8 inches focal length, is mounted at one end of 
a base-board. At the other end of the board is fixed a wooden 
upright C with a circular opening. The metal plate D rests against 
that face of the upright which is furthest from the lens and is held 
in position by a rubber band or a spring. The lens is adjusted on 
ApS B 
Fig. 4. 
the board so that the focal plane of the lens accurately comcides 
with the face of C against which D rests. The stop S should have 
a bevelled edge. A screw clamp fixed to the underside of the 
base-board allows the arrangement to be attached to an iron rod 
carried by a heavy base. ; eof 
If the focal length is measured by the goniometer method, a 
